Assad is still standing, but he is not standing alone -- and he likely no longer makes decisions alone either.

Since 2012, the Syrian regime has drawn from its allies to create a force of foreign combatants that have become essential to its survival. The presence of these fighters is a major factor in the conflict and will have significant influence on the eventual military or political solution. At this point, President Bashar al-Assad probably cannot decide the regime's course for the war on his own -- like Rome inviting the barbarian tribes to defend its gates, he has effectively mortgaged his independence to his Iranian, Hezbollah, and Iraqi allies, and their withdrawal would likely turn the war back against the regime. According to Assad's narrative, the Syrian Arab Army is winning the fight against the rebels, but it is the foreign legions that have made such claims possible.

WHY IS THE REGIME USING FOREIGNERS?

Several factors have led to the presence of foreign forces in Syria. First, the dynamics of the war created a need for large numbers of additional infantry. Based on reported casualties and the estimated effects of defection, desertion, and unreliability, the regime's regular forces have been whittled down from over 300,000 to perhaps less than 100,000, with even fewer available for combat operations. Data from the Syrian Observatory for Human Rights (SOHR) indicates that some 37,000 regular forces have been killed in combat, and the actual number is likely thousands higher. Many more have been wounded. Defections, especially from the army, have further reduced the available manpower, as has the unreliability of Sunni personnel whom the Alawite regime has asked to help pacify the country's largely Sunni populace.

The regime has responded to the decline of its regular forces by creating a variety of irregular forces (shabbiha militias, Popular Committees, local militias, the Baath Party Militia) and then merging them under the so-called National Defense Forces. According to Israeli estimates, Damascus has already generated over 50,000 personnel for this umbrella force and is aiming for 150,000.

The ready availability of forces from regime allies has been a factor as well. Hezbollah fighters, Iraqi Shiites, Palestinians, and Iran have produced some of the manpower Assad needed. As with regime irregulars, these foreign forces have become more effective in their military roles over time. Combat operations in places such as al-Qusayr, Aleppo, and Damascus have made clear that the regime is more successful where its foreign allies are present than where they are not. Without them, the regime would likely be unable to undertake significant ground offensives at this point in the war, and it would have difficulty defending some areas of the country where it is still holding on.

MAKEUP OF THE LEGIONS

The two main groups of foreigners fighting for the regime are Hezbollah personnel and Iraqi Shiites. They provide the extra combat power the regime requires. Hezbollah has been the most important contributor because of the wider scope of its activities, but the Iraqi role has been significant and appears to have expanded over time.

Hezbollah probably maintains around 4,000 men in Syria at any one time, and it has likely rotated much larger numbers through the country -- perhaps as many as 10,000. The group's fighters are found on all key battlefronts, and it plays an important role in training, advising, and bolstering regime regulars and irregulars. It was instrumental in the regime's victory at al-Qusayr last spring, in the defense of Damascus and Aleppo throughout much of 2013, and in offensive operations in Aleppo province and the Damascus suburbs later in the year. The casualties Hezbollah has suffered testify to its deep involvement: at least 300 have reportedly been killed (probably many more) and hundreds wounded.

Iraqi Shiite fighters are also present in large numbers, joining a number of different Shiite formations and often fighting alongside Hezbollah. Originally centered in the Damascus area -- especially the Sayyeda Zainab shrine, but also the broader southern suburbs and East Ghouta -- they can now be found on other battlefields, including Aleppo and Qalamoun. Multiple Iraqi-associated combat units have been identified in action, the Abu Fadl al-Abbas Brigade being the most cited. Like Hezbollah, the Iraqis are taking significant casualties, with around 300 killed based on SOHR reporting.

Palestinians have played a much smaller role. Members of the Popular Front for the Liberation of Palestine-General Command have fought on the regime's side in refugee camps around Damascus, especially the hotly contested Yarmouk camp in the southern suburbs.

Iran's role has been primarily as a coordinator and facilitator of foreign forces in Syria; its direct role in combat appears to be quite limited. Tehran encouraged -- and perhaps dictated -- Hezbollah's decision to intervene directly in the war, and it has helped Iraqi Shiite fighters with training, movement, and arms. In addition, it provides military assistance, advice, and technical expertise to the regime, in some cases by deploying military specialists on the ground. Although Iran does not appear to have committed large numbers of combat troops, personnel from the elite Islamic Revolutionary Guard Corps-Qods Force are operating in Syria, and some have been killed.

Small numbers of other foreigners have been reported on the regime's side as well, including Yemeni Houthis, Turkish Alevis, Russian "mercenaries," Afghans, Pakistanis, and even North Korean. But the evidence for this is limited, and in any case their effect on the fighting would be minimal. Overall, Israeli and other estimates indicate that something like 10,000 foreigners are currently fighting for the regime, though the figures are soft.

ROLES, OPERATIONS, EFFECTS

The importance of the foreigners lies in their effect on the war, not in their numbers. Allied fighters perform a wide variety of roles for the regime. Hezbollah and Iranian personnel are reportedly involved in determining the war's strategic direction and the conduct of combat operations -- according to a regional security source, they have formed a joint command center with the regime.

On the ground, foreigners provide critical military capabilities, especially reliable light infantry. Their willingness to fight has been crucial to regime offensive and defensive operations in Homs province (al-Qusayr), the Damascus suburbs, and Aleppo province and city.

In addition, Hezbollah and Iranian Qods Force personnel have reportedly played a key role in training regime regular and irregular forces for counterinsurgency and urban operations. This has allowed the regime to raise thousands of irregular forces to compensate for casualties among its regular units, and to reorient regular forces to a counterinsurgency mission.

Without the foreign legions, the regime's ability to conduct the war would likely have declined dramatically and perhaps decisively in 2013. Instead, its downward trend was arrested and its military fortunes improved. The synergy among regime regular and irregular forces and the foreigners has fostered success on the battlefield. Rebel forces have suffered defeats in Homs and Aleppo provinces and the Damascus suburbs, and their combat losses have increased. When the regime chooses to commit significant firepower and combat forces, including foreigners, the rebels have not been able to respond effectively. And while the foreigners have suffered casualties, especially Hezbollah and the Iraqis, the losses seem manageable thus far.

IMPLICATIONS

The role of foreign Islamist extremists in the Syrian opposition has tended to obscure the role of foreigners on the regime side. Yet there are likely as many or more foreigners fighting with Assad than against him. This issue should be addressed in the international Geneva II negotiations currently under way, especially if the regime attempts to focus the discussions on "foreign terrorists" supposedly conspiring with Western and regional governments against Syria.

Assad's foreign legions have been instrumental in keeping the regime in the war. They are not the only factor that has done so -- the regime's internal mobilization and the provision of financial assistance, diplomatic support, and weapons from its allies have played major roles as well -- but they are important. The foreigners have helped prolong the fighting and changed the slope of the battlefield in the regime's favor. Their departure would be a major blow to the regime, arguably more damaging than the departure of Sunni foreign fighters would be to the rebels. They also give Iran and Hezbollah a voice in the military and political processes that will eventually end the war. In short, Assad is still standing, but he is not standing alone -- and he likely no longer makes decisions alone either.

Jeffrey White is a Defense Fellow at The Washington Institute and a former senior defense intelligence officer.