Introduction

Foreign military interventions in ongoing civil wars are inherently problematic and increase both the complexity and the intensity of the conflicts at hand (Pearson 1974; Gleditsch and Beardsley 2004; Hironaka 2005; Lacina 2006; Lounsbery 2016; Jenne and Popovic 2017; Abu-Bader and Ianchovichina 2019; Stein and Cantin 2021). These problems tend to be less severe whenever the armed intervention is undertaken by a single outside state, since that state can usually impose a degree of orderliness on the tactical operations that ensue (Regan 2000). Interventions carried out by multiple states, by contrast, can be expected to be much less coherent, either because the individual interests of the intervening states interfere with the common enterprise (Cunningham 2010) or because the interveners vie with one another on the battlefield (Mitton 2017).

Furthermore, foreign military interventions prolong internal wars (Elbadawi and Sambanis 2000; Brandt, Mason, Gurses, Petrovsky and Radin 2008; Dixon 2009; Regan 2010; Linebarger and Enterline 2016). Why this happens has been addressed in a variety of ways. The deployment of outside troops diminishes the difference in relative capabilities between the pro-government coalition and challengers to the existing order, making it more difficult for either side to achieve a decisive victory (Balch-Lindsay and Enterline 2000; Lockyer 2011; Sullivan and Karreth 2015). Weaponry that foreigners deliver to any one party in the conflict tends to be captured or otherwise acquired by that party's local adversaries, enhancing their capacity to continue fighting (Collier et al. 2004). The presence of external forces may lead the combatants to postpone engaging in serious negotiations (Mason, Weingarter and Fett 1999).Footnote 1 In addition, foreign involvement frequently provokes a patriotic response, whereby local forces mobilize to resist the infringement on homeland sovereignty that is associated with the arrival of outsiders (Edelstein 2008; Lyall and Wilson 2009). To what extent these disparate explanatory threads can be knitted together to cover important instances of intervention remains an open question. As Jeffrey Dixon (2009, 131) observes, ‘the exact process by which [external] military interventions prolong [internal] wars is poorly understood.'Footnote 2

Why foreign military interventions prolong civil wars

David Cunningham (2010) offers arguably the most cogent account of why foreign military intervention lengthens civil wars. In his view, outside actors bring to the conflict one or more ‘independent objectives' that further their own interests (Cunningham 2010, 116). The introduction of such extraneous goals makes it much more difficult for the warring parties to reach an agreement that might bring the fighting to a close, for four main reasons. First, ‘the addition of [unrelated] issues complicates the bargaining environment because it shrinks the "bargaining range" of acceptable agreements that all combatants could prefer to continued warfare' (Cunningham 2010, 117). Second, with more participants it becomes ‘harder for combatants to use battle outcomes to update their beliefs about the likelihood that they will win the conflict' (Cunningham 2010, 117), and the prevalence of imperfect information engenders further fighting (Walter 2009). Third, since each party has a strong incentive to avoid committing itself to a prospective agreement until the very last moment, a greater number of combatants will extend the time it takes to conclude negotiations (Cunningham 2010, 118). And fourth, a larger number of actors increases the chances that one actor or another will refuse to accept or comply with the terms of any provisional deal (Cunningham 2006).

Besides whatever conceptual incompatibilities might exist among these four hypotheses, Cunningham's analysis suffers from a pair of notable shortcomings. On the one hand, it conflates two distinct lines of argument concerning the ‘independent objectives' that foreign interventions inject into an ongoing civil wars. The first—and the one that Cunningham (2010, 119) himself emphasizes—is that any single outside state usually brings one or more of its own ambitions to the conflict, making a negotiated resolution harder to achieve. The second is that whenever multiple states intervene, their respective goals and priorities are apt to clash, making mutually acceptable settlements elusive (Cunningham 2010, 117). In order to tease these two arguments apart, one would need to explore cases in which a single external state intervenes, and counterpose those cases against instances of multistate intervention.

On the other hand, Cunningham focuses primarily on the intentions—or perhaps more accurately, the motivations—of the intervening states, rather than on developments that occur as a result of their intervention. What matters most in his view is the extraneous goals that foreign governments harbor, not the impact that outside involvement has on the trajectory of the civil war. So long as interveners add to the mix ‘diverse preferences over the outcome of the war,' the conflict is presumed to drag on (Cunningham 2010, 119). Yet motivational factors only matter to the extent that they affect the actual fighting on the ground. In the case of the Congo from 1996 to 2002, for instance, Cunningham (2010, 119) observes that Rwanda and Uganda formed partnerships with an assortment of competing anti-government forces, and it was this aspect of their involvement that prolonged the conflict. The extent to which outside states sponsor rival movements and organizations inside the war-torn country thus seems more pertinent to the question of why civil wars persist than does any of the four hypotheses that Cunningham adduces.

Revisiting the impact of foreign military interventions

Outside states almost always collaborate with influential internal combatants whenever they deploy military forces into an ongoing civil war. Such alignments provide the intervening state with significant benefits. They enable the intervener to send fewer troops into combat than might be required if it tried to carry out the campaign by itself. They supply foreign commanders with local knowledge that would be difficult, if not impossible for newly arrived outsiders to acquire quickly and reliably enough to be of use. They provide a measure of legitimacy for the presence of foreign troops inside the embattled country's territory. And they send a strong signal to actual and potential external adversaries, as well as to any internal actors who might be wavering about which side to join, that the situation has shifted to favor the intervener.

Forging alliances with internal combatants nevertheless creates notable difficulties for the intervening state. To the extent that local allies retain a sufficient degree of autonomy to undertake tactical operations on their own initiative, the dynamics associated with principal-agent problems are likely to take shape (Salehyan 2010; Coletta 2013; Salehyan et al. 2014). In particular, local allies ‘may shirk [their] responsibilities and devote suboptimal effort to the war while privately consuming the [intervening state's] resources'; they might ‘prove incompetent and fail to effectively challenge' the appropriate adversaries; and they ‘may engage in egregious behaviors that are either contrary to the goals and strategic interests of the [intervening state] or that generate domestic and international backlash against it' (Salehyan et al. 2014, 639). Furthermore, whatever efforts the intervening state makes to strengthen its connections with local allies give those allies the opportunity to carry out initiatives that have the potential to entrap the intervener in situations it would prefer to avoid (Snyder 1984). These problems will be more pronounced whenever the intervening state aligns itself with more than one internal combatant, each of which harbors preferences and objectives of its own.

Difficulties inherent in foreign intervener–local ally relations become compounded if the intervention involves more than one intervening state. In the first place, the interveners are likely to form alliances with different local combatants, on the basis of divergent strategic and tactical considerations. Second, collective action problems make it harder for several intervening states to compel local allies to keep their attention and activities riveted on the common endeavor. Third, ‘agents with multiple principals are less constrained in their behavior than those with a single principal. A single principal is able to give clear directives and the agent reports to a sole actor. In cases where the [local ally] has multiple principals, however, [it] can play these actors off against one another to extract a better bargain on more favorable terms' (Salehyan et al. 2014, 643). Consequently, local allies of more than one foreign intervener have both the incentive and the capacity to put their own interests ahead those of the putative principals.

These factors provide the basis for four analytically distinct types of foreign military intervention. The first is one in which a single intervening state aligns with a single local combatant—or in rare instances a solidly united coalition of local combatants. In the second, a single intervening state forges partnerships with several local combatants. The third is one in which more than one outside state intervenes, and these states join forces with a single domestic combatant. And in the fourth, multiple external interveners ally with a number of internal combatants (see Fig. 1).

Fig. 1
figure 1

Types of foreign military intervention

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Other things being equal, Type I interventions can be expected to exhibit the fewest and most manageable principal-agent and collective action problems. A comparatively high level of coordination between the intervening state and the local ally will enable the two of them to carry out coherent combat operations, as well as to negotiate with adversaries in an effective way. Civil wars characterized by Type I interventions are therefore likely to end relatively quickly. It seems possible that the exceptionally short duration of the 1957 civil war in Oman (2 months) and the 1981 civil war in Gambia (1 month) can be attributed to the involvement in each case of one outside state, which aligned with only one internal combatant (Lawson 2019, 238).

Type IV interventions, by contrast, will be characterized by a wide range of principal-agent and collective action problems, in addition to profound dilemmas concerning alliance management. Divergences of interest and action among the multiple intervening states and the various local allies will not only interfere with the orderly and effective prosecution of tactical operations, but also make it much more difficult to reach a mutually acceptable bargain with adversaries. Type IV interventions will consequently accompany, if not actually generate, the lengthiest civil wars. The prolonged character of the 1975–91 civil war in Lebanon (196 months) and the 1975–2002 civil war in Angola (316 months) no doubt reflect the dynamics inherent in this type of intervention (Lawson 2019, 239).

Type II interventions can be expected to be less orderly than Type I interventions and to exhibit a substantial amount of rivalry among local allies. Since each of the internal combatants has the capacity and incentive to engage in activities that can entrap the intervening state, as well as to disrupt negotiations and scuttle proposed settlements, civil wars characterized by Type II interventions will drag on for a considerable time. Mozambique's 1976–92 civil war (191 months) is one of the few contemporary examples of this type of intervention (Lawson 2019, 238).

Type III interventions may well involve a high degree of strategic and battlefield rivalry among the intervening states, but such rivalry will have a less detrimental impact on developments on the ground than does jockeying for influence and position among multiple local allies. Civil wars that attract Type III interventions are thus likely to have shorter durations than ones with Type II interventions. The 1992–98 civil war in Tajikistan (68 months) belongs in this category (Lawson 2019, 239).

It is entirely possible that any particular civil war will shift from one type of foreign military intervention to another as time passes.Footnote 3 Additional outside states may decide to intervene in the fighting; new local combatants may emerge, and existing combatants may splinter to create mutually antagonistic armed formations. As a provisional hypothesis, it seems more likely that Type I interventions will shift to Type II interventions than they will to Type III interventions. The number of intervening states is most often set at the start of the intervention and only increases after more internal combatants join the battle. How frequently, and in which directions, shifts take place from one type of interventions to another constitutes an important area for future exploration.

Further investigation will also be necessary to tease out two sets of conceptual nuances regarding foreign military interventions that are not always kept clear in existing research. One concerns the distinction between multiple-state interventions and multilateral interventions. The latter term refers to campaigns undertaken by more than one state under the auspices and supervision of an international or regional organization, such as the United Nations or the Economic Community of West African States. Multilateral interventions can be expected to exhibit a high degree of strategic and tactical unity, as well as firm commitment to a common objective. It would be useful to determine whether the dynamics and outcomes of multilateral interventions more closely resemble those that characterize single-state interventions or those associated with multiple-state interventions.

Another conceptual subtlety regarding foreign military interventions that has become increasingly pertinent concerns the extent to which the intervening state(s) act as the agent(s) of a great power, which provides the armaments, training and battlefield intelligence to support the campaign.Footnote 4 It would be a serious mistake to assume that non-great power interveners usually serve as agents or proxies for one or more of the great powers (see Boussaid 2021). Still, it occasionally does happen that a regionally influential state steps in to carry out a task that reflects the agenda and interests of a global power. Whether or not this kind of cross-regional patron–client relationship affects the dynamics and outcomes of interventions in civil wars merits careful attention.

As a plausibility probe of this analytical scheme, it is useful to examine the trajectory of a notable instance of Type IV foreign military interventions: the campaign orchestrated by the Kingdom of Saudi Arabia to rescue the government of the Republic of Yemen from an armed challenge rooted in the country's northern marches. The most recent outbreak of this recurrent conflict occurred during the spring of 2012, although fighting remained sporadic and restrained for the first two years. Extensive warfare was still taking place as of October 2021, marking some 115 months of continuous combat—substantially longer than the 77-month average duration of civil wars in which a single outside state intervenes militarily and just six months less than the average for civil wars in which more than one external state intervenes (Lawson 2019, 235).

Foreign military intervention in Yemen's civil war, 2015–18

Throughout 2013–14, the threat to Saudi Arabia emanating from the civil war in Yemen steadily intensified (al-Muslimi 2015a; Alyahya 2015; Legrenzi and Lawson 2016; Hokayem and Roberts 2017). The authorities in Riyad charged that the Islamic Republic of Iran was sponsoring the activities of a movement based in Sa'dah province called the Supporters of God (Ansar Allah, commonly known as the Huthis), the most potent of a handful of challengers to the government headed by President 'Abd Rabbuh Mansur Hadi (Terrill 2014; Salisbury 2015; Zweiri 2016; Juneau 2016). The Supporters of God launched a large-scale offensive into the adjacent province of 'Amran in April 2014, which inflicted a severe defeat on both the government-aligned, mainstream Islamist party called the Yemeni Congregation for Reform (al-Tajammu' al-Yamani li al-Islah) and elite regular army units commanded by General 'Ali Muhsin al-Ahmar. The defeat precipitated a grudging rapprochement between the Congregation for Reform and Saudi Arabia, which was facilitated by the rise of a new leadership in Riyad that was less antagonistic toward the party's regional allies, the Muslim Brothers (al-Ikhwan al-Muslimin) (Hedges and Cafiero 2017). It also led to an unprecedented alignment between the Supporters of God and regular army units that had remained loyal to Yemen's pre-2011 president, 'Ali 'Abdullah Salih (Schmitz 2014).

Five months later, the Supporters of God and troops loyal to Salih seized control of the capital city, San'a. They then advanced southward toward the port city of al-Hudaidah on the Red Sea coast and the populous central provinces of Dhammar, al-Baida, Ibb and Ta'izz. This offensive, along with President Hadi's abrupt resignation in late January 2015, prompted Riyad to intervene in the fighting. That March the Saudi air force launched a succession of attacks against the anti-government alliance, joined by air, sea and land forces of the United Arab Emirates (UAE), Kuwait, Bahrain, Qatar, Sudan, Jordan and Morocco. The bombing campaign and concurrent landing of UAE infantry and armored units on Yemen's southern coast prevented the Supporters of God from consolidating its hold over the southern metropolis of Aden (Knights and Mello 2015). At the same time, Saudi-sponsored Islamist militants, some affiliated with the Congregation for Reform and others aligned with the Hadi government, checked the Supporters of God's offensive just south of Ta'izz city (al-Sakkaf 2015).

Initial intervention and the rise of new combatants

Fierce resistance on the part of the Supporters of God and Salih loyalists blocked UAE troops and regular army units loyal to the Hadi government from pushing northward from Aden into the adjacent provinces of Lahij, Abyan and al-Dali’.Footnote 5 Consequently, the Saudi-led coalition's ground campaign bogged down. The stalemate allowed Islamist militants affiliated with the Islamic State (al-Dawlah al-Islamiyyah) to join the fighting, some of whom were reported to have coordinated their activities with Saudi and UAE commanders (al-Muslimi 2015b; Rafi 2015; al-Hammadi 2016). By collaborating with militant Islamists, UAE troops were able to push the Supporters of God out of portions of Lahij and Abyan, although friction between the newly arrived Islamic State and the Yemeni branch of al-Qa'idah, called the Supporters of the Islamic Way of Life (Ansar al-Shari'ah, also known as al-Qa'idah on the Arabian Peninsula or AQAP), precipitated a flurry of attacks not only against UAE troops but also against UAE-sponsored local defense companies (Naylor 2015; al-Falahi 2015; middleeasteye.net 2015).

After establishing a foothold in Lahij and Abyan, the Islamic State and other local Islamist militants moved northward into the sparsely populated provinces of Shabwah, Marib and al-Jawf, on the border with Saudi Arabia. The Supporters of the Islamic Way of Life meanwhile strengthened its position in al-Baida and the eastern province of Hadramawt (al-Dawsari 2018). Remnants of Yemen's regular army stationed in Marib prevented the militant Islamists and the Supporters of God from taking charge of the extensive wastelands along the Saudi–Yemeni frontier, but Saudi-sponsored forces proved less successful in dislodging the Supporters of God and Salih loyalists from al-Baida, al-Dali’, Ibb and Ta'izz (al-Jazeera 2015; Schmitz 2016). Unable to gain ground in the central highlands, pro-government and UAE-backed forces dug in around Aden and al-Hawtah, the capital of Lahij province, where they were subjected to sporadic attacks by Islamist militants (Agence France Presse 2016a, 2016b; Alwly 2016; Ghobari and Bayoumy 2016). More importantly, disagreements between Saudi and UAE commanders concerning which militants to placate enabled the Islamic State and the Supporters of the Islamic Way of Life to expand throughout al-Baida, Shabwah and Hadramawt during the winter of 2015–16 (Arrabyee 2016a; al-Dawsari 2018; Kendall 2018).

By early 2016, UAE troops and pro-government forces had started to collaborate with cadres of the Southern Movement (al-Hirak al-Janubi), whose platform—which called for the restoration of political autonomy in Yemen's southern and eastern provinces—fit uneasily with the security interests of Saudi Arabia (International Crisis Group 2016; Partrick 2016). The leverage that the Southern Movement gained from its alignment with the UAE aggravated tensions inside the Saudi-led coalition, and accorded the Supporters of the Islamic Way of Life the opportunity to capture al-Hawtah and Zinjibar, the capital of Abyan province, as well as to overrun the pivotal Aden suburb of al-Mansurah (International Crisis Group 2016, 19; Ghobari and Bayoumy 2016). President Hadi attempted to restore unity among his disparate partners in early February 2016 by appointing moderate southerners to senior administrative positions (al-Falahi 2016), but this initiative did little to brighten the government's prospects (Nasser 2016; Abdul-Ahad 2016; Arrabyee 2016b).

Intervener–ally operations and intra-coalition fragmentation

UAE troops carried out a series of joint operations with southern-based forces during the spring of 2016, which expelled the Islamic State and the Supporters of the Islamic Way of Life not only from al-Hawtah and Zinjibar, but also from the port cities of al-Mukalla and al-Shihr in Hadramawt province. Following these victories, the leadership of the Southern Movement proclaimed its intention to ‘disengage' the territory under its control from the Republic of Yemen (al-Akhbar 2016a). Calls for southern autonomy accompanied popular protests in Aden over the Hadi government's inability to maintain a reliable supply of electricity. UAE commanders meanwhile organized a new military formation to patrol Hadramawt, called the Hadrami Elite Forces (Quwwat al-Nukhbah al-Hadramiyyah), and stood aside as southern activists convened a General Congress that demanded the creation of an ‘independent province' in southern and eastern Yemen. A counter-congress that gathered in Riyad, organized by pro-Saudi notables from Hadramawt, fell flat (al-Akhbar 2016b; El Yaakoubi 2017).

Rising calls for southern autonomy created a rift between the supporters of Hadi's government and forces aligned with the UAE, particularly the network of local defense companies that made up the Security Belt (Hizam al-Amn) surrounding Aden (middleeasteye.net 2017a). These comparatively well-equipped formations operated outside the regular army's command structure and assumed a crucial role in protecting the southern metropolis's harbor, airport and major roads. At the same time, UAE commanders set up bases on Greater Hanish (or Perim) Island, in the heart of the Bab al-Mandab Straits, and Suqutra Island, in the Arabian Sea southeast of al-Mukalla, even as UAE-based enterprises stepped up investments in infrastructure and commercial property on Suqutra (Samir 2017).

President Hadi took steps in May 2017 to shore up his deteriorating position vis-à-vis the Southern Movement and its UAE enablers, dismissing prominent figures in the Movement from their ministerial posts and provincial governorships. The ousted officials formed a Transitional Council and declared that it intended to ‘manage the provinces of the south and represent them domestically and internationally’ (Agence France Presse 2017a; al-Hammadi 2017a; Forster 2017). The Transitional Council gained strength as protests against Hadi's leadership picked up momentum throughout the south, even though a number of leaders of the Southern Movement refused to join the new body (Mukhashaf 2017; Agence France Presse 2017b; Agence France Presse 2017c; al-Hammadi 2017b; Al Qalisi 2017; Forster 2017: 137).

Islamist militants took advantage of the rift between the Hadi government and the Southern Movement to recover parts of al-Dali', al-Baida and Abyan provinces that they had lost during the spring and summer of 2016. In a few districts, the Supporters of the Islamic Way of Life coordinated operations with the Supporters of God and troops loyal to former President Salih (Khan 2017). Nevertheless, tribal forces recruited by the UAE pushed the militant Islamists out of a key district on the border between al-Baida and Shabwah provinces in early August 2017 and then took charge of the highway linking Shabwah to Saudi Arabia. These initiatives prompted prominent figures in the Southern Movement to charge that the UAE was pursuing its own interests to the detriment of the local populace (al-Sharq 2017). That October, activists affiliated with the Congregation for Reform began to assassinate pro-UAE preachers in Aden. Members of UAE-sponsored defense companies were rumored to have been encouraged to retaliate by attacking offices of the Congregation for Reform and targeting preachers who voiced criticism of the UAE (middleeasteye.net 2017b; al-Naqib 2017; al-Husni 2017).

Saudi commanders responded to the UAE's growing assertiveness in Yemeni affairs by stepping up shipments of heavy weaponry to forces aligned with President Hadi, including some formations affiliated with the Congregation for Reform and others linked to the Supporters of the Islamic Way of Life (middleeasteye.net 2017c). This course of action contravened not only a basic tenet of the Saudi–UAE partnership, viz. the two states' shared antipathy toward the Muslim Brothers, but also the UAE's expressed hostility toward the Congregation for Reform. Burgeoning antagonism between Saudi-sponsored forces and UAE-backed units opened the door to renewed attacks by Islamist militants against government buildings in Aden (al-Jazeera 2017). Meanwhile, the partnership between the Supporters of God and Salih loyalists crumbled: Armed clashes erupted between the two groups in the streets of San'a in early December 2017 (Al-Mujahed and Fahim 2017; Mohamed and Al Qalisi 2017; Al Wasmi and Mahmood 2017). Salih declared that he would be willing to defuse the crisis by engaging in talks with the authorities in Riyad, but on 4 December he and one of his nephews—a former commander in the elite Republican Guard—were executed by cadres of the Supporters of God.

Moves toward intra-coalition reconciliation

While fighting raged in the capital during the first weeks of 2018, a Saudi-sponsored militia, the Tihama Resistance (Muqawamah al-Tihama), captured the al-Khukhah district on the Red Sea coast, Riyad's first significant ground victory of the entire intervention. Saudi warplanes at the same time bombed Supporters of God positions in the northern province of Hajjah (Mahmood 2017a). More importantly, Saudi forces advanced into the far eastern province of al-Mahrah, after its governor refused to acknowledge the authority of the Transitional Council (Nagi 2018).

As the Saudis gained ground in the west and the far east, the de facto commander of the UAE armed forces, Muhammad bin Zayid Al Nuhayyan, told reporters that a negotiated solution to the conflict would not be permitted to happen ‘at the expense of the security and stability of the region, and it won't be at the expense of allowing militias to work outside the scope of the [Yemeni] nation' (Nagi 2018). Armed tribespeople affiliated with the Southern Movement subsequently resumed their offensive against Islamist militants in Shabwah. At the same time, regular army units aligned with President Hadi that had been transported from the south to al-Jawf province pushed into the mountains just east of San'a (Salisbury 2017, 23; Mahmood 2017b). These parallel operations provided the context in which Muhammad bin Zayid held unprecedented talks in the Saudi capital with senior figures of the Congregation for Reform, in the presence of Saudi Crown Prince Muhammad bin Salman Al Sa'ud. The Congregation for Reform's leader reported afterward that he had been assigned the task of encouraging Salih loyalists to switch sides and join the campaign against the Supporters of God (Hubbard and Youssef 2017).

UAE troops took up positions alongside Saudi-backed forces in Ibb and Ta'izz in January 2018, and working together disrupted vital supply lines connecting Supporters of God strongholds all across the central highlands. Saudi and Emirati officials then tried to install former President Salih's nephew, Tariq 'Abdullah Salih, as a unifying figure in the south, but the scheme provoked outrage among members of the Southern Movement. Widespread popular discontent prompted several local defense companies that were less dependent on the UAE to erect barricades and checkpoints throughout Aden (al-Akhbar 2018a). Clashes soon broke out among partisans of southern autonomy, pro-UAE formations and regular army units affiliated with Hadi, while Southern Movement activists took to the streets to demand the ouster of Prime Minister Ahmad 'Ubaid Bin Daghir (middleeasteye.net 2018a; Al-Mujahed and Raghavan 2018; Dahlgren 2018). Violence escalated after pro-Hadi troops opened fire on the protesters and Southern Movement fighters retaliated by assaulting regular army installations.

Islamist militants took advantage of the chaos in Aden to resume attacks against pro-government forces and local defense companies in al-Baida, al-Dali' and Shabwah. Besieged in Aden and facing renewed pressure in the adjacent provinces, President Hadi ordered the state's land survey department to block Emirati enterprises from acquiring more property on Suqutra Island (al-Hammadi 2018a). This initiative directed against the UAE was facilitated by a handful of battlefield successes on the part of Saudi-backed, pro-Hadi forces in Ta'izz, al-Hudaidah and Sa'dah provinces (Mahmood and Al Wasmi 2018).

Despite the animosity between Hadi's government and UAE-sponsored formations, UAE troops joined Saudi-backed fighters that February in an offensive against the Supporters of the Islamic Way of Life in Hadramawt. Saudi and UAE commanders subsequently announced that Tariq 'Abdullah Salih was going to be put in command of a new formation operating out of the Red Sea port of al-Mukha, which would be designated the Republican Guard (Haras al-Jumhuriyyah) and constitute a central pillar of the newly created National Resistance Forces (Quwwat al-Muqawamah al-Wataniyyah) (Mahmood 2018a; al-Mashhad al-Yamani 2018; al-Akhbar 2018b; Browne 2018). Fighters with links to the Southern Movement joined the National Resistance Forces as the Giants' Brigade (Liwa al-'Amaliqah) (Salisbury 2018). Resuscitated collaboration between Saudi-backed and UAE-sponsored forces along the western coast accompanied a sharp rise in antagonism in the hills around Ta'izz between fighters affiliated with the Congregation for Reform and local Islamist militants, particuarly the battalion led by 'Adil 'Abdu Far'i (known as Abu al-'Abbas) that had thrown its lot in with the UAE (al-Akhbar 2018c; al-Maqtari 2017).

Fighting among local allies and intra-coalition fragmentation

Violence surged once again in and around Aden during the late winter and spring of 2018. Suicide bombers carried out a series of attacks against the Southern Movement, along with numerous assaults on components of the Security Belt. In response, preachers associated with the Congregation for Reform were subjected to a wave of assassinations. The reciprocal killings dampened popular sympathy for the UAE and its partners (middleeasteye.net 2018b; Associated Press 2018; Raghavan 2018; Kendall 2018, 23–24), and led a group of prominent figures to organize the Southern National Coalition (Ihtilaf al-Watani al-Janubi), which was designed to replace the Southern Movement with a broader grouping of parties that might be able to negotiate more productively with the Hadi government (al-Hammadi 2018b). These developments prompted Prime Minister Bin Daghir to take steps to regain control of Suqutra, but his efforts convinced UAE commanders to dispatch additional troops and warplanes to the island (al-Hammadi 2018c; middleeasteye.net 2018c; Michaeli 2018). Hostility between pro-government and UAE-sponsored forces intensified when regular army units commanded by General al-Ahmar arrived in Aden from Marib, reportedly at the instigation of President Hadi (al-Akhbar 2018e). At the same time, fighting flared between the Islamic State and the Supporters of the Islamic Way of Life in al-Baida and Abyan, as well as between various militant Islamist formations and UAE-sponsored forces in Shabwah (Mahmood 2018b; Kendall 2018, 9–10 and 20–21).

Renewed combat across the south, combined with stalemate in the west, redoubled antagonism between the Hadi government and forces aligned with the UAE. By late July 2018, the latter had all but imprisoned senior government officials inside their residences in Aden (al-Hammadi 2018d). Festering animosity between proponents of southern autonomy and the central government was evident not only in a roadside bombing on the outskirts of Aden that targeted a high-ranking state security commander and but also in an attack on the graduation ceremony at the military academy in the heart of the metropolis (Mahmood 2018c; Reuters 2018; Abdu 2018). In late August, the battalion led by Abu al-'Abbas pulled out of the battle to recapture Ta'izz, following clashes with pro-government and Congregation for Reform-affiliated fighters (al-Hammadi 2018e; middleeasteye.net 2018e). As summer turned to autumn, roiling popular discontent over the presence of Saudi troops precipitated a series of protests in al-Mahrah (middleeasteye.net 2018f; al-Jazeera 2018).

Widespread disaffection in the south—sparked by pervasive official corruption, the soaring cost of living and the scarcity of opportunities for meaningful employment—undercut both the Hadi government and the UAE's expeditionary force. One widely respected leader of the Southern Movement who had kept his distance from the Transitional Council and the Southern National Coalition alike charged that the Council was nothing but a puppet of the UAE. Moreover, he asserted that the UAE had now become an occupying power, which ‘could not possibly express the south's [authentic] voice. It rather expresses its own greed and interests in [controlling] the island of Suqutra and 1500 km of the Arabian Sea coast, the source of [most of Yemen's] oil and the ports' (al-Khalij 2018). This outburst of blatantly anti-UAE sentiment resonated throughout Abyan and Shabwah, and became pronounced in Ta'izz city, where protests broke out that October against ongoing Saudi and UAE air and ground operations (al-mayadin.net 2018).

Implications of the Yemen case

Foreign military intervention in Yemen from the spring of 2015 to the autumn of 2018 illustrates four ways in which the involvement of multiple intervening states in conjunction with several local allies extends the duration of an ongoing civil war. First, Saudi Arabia and the UAE injected into the conflict incompatible strategic and tactical objectives, which interfered with the effective prosecution of the collective project. Second, the Saudis and Emiratis allied themselves with rival forces on the ground, whose respective leaderships refused to subordinate their ambitions to the common task of defeating the Supporters of God. Third, rivalry between the local allies of Saudi Arabia and those of the UAE created opportunities for additional combatants to enter the fighting, whose activities enlarged the field of battle, required greater effort and more resources on the part of the interveners, and necessitated different kinds of tactical operations. And fourth, external intervention diminished the influence and legitimacy of the Hadi government, and prevented it from acting as the primary interlocutor of the pro-government coalition.

To what extent the objectives of Saudi Arabia and the UAE differed at the outset of the intervention (Eleftheriadou 2021), or whether they instead diverged during the course of the military campaign, is hard to determine. Officials in Riyad consistently took it upon themselves to articulate the purposes and priorities of the coalition as a whole, while Emirati officials usually kept their own counsel. Early public statements by the two governments were virtually indistinguishable from one another (Abdullah 2015; International Crisis Group 2016, 23–24). By the winter of 2015–16, however, the gap between these states' overriding goals had become apparent (Diwan and Abo Alasrar 2016); it widened further after UAE troops took control of al-Mukalla and al-Shihr (Rai al-Yawm 2017; al-Akhbar 2018d). Following the death of 'Ali 'Abdullah Salih in December 2017, the divergence grew even more pronounced (Partrick 2017b; Ardemagni 2018).

On the whole, Saudi Arabia intervened in Yemen in pursuit of three strategic objectives: to prevent Islamist militants hostile to the Saudi regime from coalescing in the territory along the kingdom's southern border; to block the Islamic Republic of Iran from gaining a foothold on the Arabian peninsula; and to restore the kind of equilibrated political arrangement among influential Yemeni parties that had been envisaged in the 2013–14 National Dialogue Conference (Popp 2015; Partrick 2015). This admixture of goals left open the possibility of a reconciliation with the Supporters of God—so long as the movement kept the Yemen–Saudi Arabia border area secure, renounced its connections with Tehran and did not to try to dominate Yemeni domestic politics. It also allowed for cooperation with the Yemeni Congregation for Reform, despite that party's historic links to the Muslim Brothers. Furthermore, these objectives implied the preservation of a unified Yemen, with a centralized authority structure capable of suppressing Islamist militants and exercising control over the country's far-flung frontiers.

UAE objectives in Yemen have been more opaque. Maintaining general solidarity with Saudi Arabia constituted an underlying motive behind the initial decision to take part in the intervention, as did animosity toward Iran and a desire to eliminate threats to commercial shipping in the Red Sea and Gulf of Aden (Ibish 2017, 5–6; Ardemagni 2016). Combating the spread of militant Islamist movements, a category that from an Emirati perspective includes the Muslim Brothers, played a role in persuading the UAE to join the military campaign as well (Ibish 2017, 27; Partrick 2017a; Ardemagni 2018). It is therefore not surprising that UAE commanders shifted their attention away from the Supporters of God and targeted the Supporters of the Islamic Way of Life and the Islamic State during the early months of 2016 (Ardemagni 2016; Salisbury 2016). This shift opened the door to collaboration between UAE troops and fighters affiliated with the Southern Movement. Whether or not the forging of a tactical alliance between these two actors reflected a shared interest in promoting southern autonomy remains an open question.

Significant divergences among interveners with regard to strategic objectives thus accompanied the construction of alliances with combatants inside Yemen. The underlying tension between Saudi Arabia and the UAE became most clearly apparent with respect to the Congregation for Reform. Saudi Arabia intervened shortly after that party pulled away from the Hadi government, and officials in Riyad convinced the UAE to join the intervention by claiming that rescuing the government would contribute to the suppression of Islamist militants throughout the Middle East and North Africa. As the months went by, however, Saudi Arabia warmed up to the Congregation for Reform, especially after that party's fighters emerged as the backbone of the pro-government coalition in Ta'izz province. UAE commanders by contrast built strong connections to local forces that harbored deep-seated animosity toward the Hadi government and the Congregation for Reform alike. Such animosity animated the Southern Movement and engendered conflict between the UAE-sponsored Security Belt and formations affiliated with the Congregation for Reform. By the autumn of 2017, these parallel currents had converged, and the leadership of the Southern Movement had begun to galvanize its supporters to combat the Congregation for Reform (middleeastmonitor.com 2017).

Military intervention by Saudi Arabia and the UAE transformed the nature of the civil war in Yemen, most crucially by heightening the influence of Islamist militants. The Hadi government had confronted a succession of southern-based Islamist movements prior to 2015, even as it battled the Supporters of God in the north (Carapico 2000; Johnsen 2013). But both the number of militant Islamist formations and the scale of their operations rose sharply in the aftermath of the Saudi-led intervention (Bayoumy, Browning and Ghobari 2016). More importantly, the Supporters of the Islamic Way of Life during the fall and winter of 2015–16 abandoned its efforts to construct effective modes of local governance in the territory under its control and turned its energies to battling pro-government and UAE-sponsored formations. The fighting became more ferocious as the Supporters of the Islamic Way of Life started to compete with newly arrived cadres of the Islamic State (Kendall 2018, 19).

Battlefield successes on the part of UAE troops and UAE-sponsored formations during 2016–17 reduced the geographical domain of the Supporters of the Islamic Way of Life. Yet as Elisabeth Kendall (2018, 17–18) points out, the diminution of this movement's field of operations accompanied the emergence of rogue cells of militants, over which its leadership exercised little if any control and whose cadres engaged in the sort of indiscriminate violence that had tarnished the public image of al-Qa'idah on the Arabian Peninsula a decade earlier. By the autumn of 2018, attacks against regular army units and UAE-sponsored formations started to pick up once again, first in Hadramawt and Shabwah and later in Abyan. The resurrection of the Supporters of the Islamic Way of Life inspired reports that UAE commanders had started to incorporate Islamist militants into the ranks of the Security Belt (Michael, Wilson and Keath 2018; Fenton-Harvey 2018; Eleiba 2018). In addition, the Supporters of the Islamic Way of Life formed unprecedented partnerships with influential tribal communities in Shabwah and al-Baida, which made it a much more potent adversary.

Finally, the Hadi government suffered a substantial loss of power and prestige as a result of Saudi and UAE military intervention, which was compounded by General 'Ali Muhsin al-Ahmar's unwillingness to put elite regular army units under the president's command. The low esteem in which the Yemeni public held the country's nominal authorities by the third year of the intervention contributed to the outbreak of massive demonstrations against official corruption and incompetence, particularly during the summer of 2018. Shopkeepers in the cities of Tarim and Saiyun in Hadramawt went on strike to protest the regular army's inability to provide local security and to demand that responsibility for policing the province be turned over to UAE-sponsored formations (Mahmood 2018d). Moreover, President Hadi by mid-2018 no longer retained sufficient authority to carry on serious negotiations with the Supporters of God (Schmitz 2018). It was even reported that the government had been compelled by the UAE to acquiesce in the upcoming ground assault against al-Hudaidah, over the president's expressed opposition to that offensive (middleeasteye.net 2018d).

Conclusion

Foreign military intervention lengthened the duration of the civil war in Yemen that took shape in the spring of 2012. Nevertheless, the explanation for this lengthening has little to do with the arguments that have been proposed by existing studies of external involvement in internal warfare. Saudi Arabia and the United Arab Emirates did bring divergent objectives to the conflict, but these differences prolonged the fighting not because they shrank the range of mutually acceptable bargains or interfered with the combatants' assessments of possible outcomes. Divergent goals instead inhibited effective battlefield operations and led the two primary intervening states to forge alliances with rival combatants on the ground. The interveners' local allies pursued agendas and interests of their own, and made little effort to coordinate their activities.

Consequently, by the autumn of 2018 the civil war in Yemen had come to involve at least nine major combatants: the government headed by President Hadi and regular army units loyal to the president, the Supporters of God, regular army units loyal to former President Salih, the Yemeni Congregation for Reform, Saudi Arabia, the UAE, the UAE-sponsored Security Belt, the Southern Movement and the Supporters of the Islamic Way of Life. Furthermore, both the Supporters of God and the Southern Movement showed signs of splitting into moderate and radical factions (Mahmood 2018e; al-Khalij 2018). Under these circumstances, reaching a mutually acceptable bargain that might bring the fighting to an end became virtually impossible (Cunningham 2006).

In addition, the actions that each one of these combatants undertook to protect itself posed a severe threat to the security of one or more of the others. More importantly, each party to the conflict confronted one or more potent adversaries at the same time that it was wrestling with one or more of its putative allies. Explicating the intersecting dilemmas associated with the combatants' concurrent efforts to maximize security and manage adversary–ally relations offers the most promising explanation for why foreign military interventions end up prolonging civil wars, in Yemen and elsewhere.