Syria’s civil war has grown ever more complex in the six years since protesters first challenged the government. President Bashar al-Assad aims to reassert control nationwide, while predominantly Sunni Arab opposition forces seek to wrest the state from him. The diverse groups making up the opposition, however, differ on their visions for a post-Assad state, with their ostensible aims ranging from liberal democracy to theocracy.

Unlike Assad and the opposition, the self-proclaimed Islamic State is intent on erasing Syria’s borders to establish a state of its own in territory spanning parts of Iraq and Syria. Kurdish militants, who have fought to establish an autonomous, if not independent, national homeland in the country’s northeast, are the group’s primary foe.

The fight has been further complicated by outside powers who have funded and armed combatants and, in some cases, backed them with air support or manpower. Outgunned by pro-regime forces, many opposition groups have aligned with jihad factions.

By 2017, tens of thousands of combatants were involved in the fighting. Up to half a million Syrians have been killed, most by pro-regime forces, and more than half the country’s prewar population of some 22 million has been displaced. The armed groups have been marked above all by flux—in their membership, capabilities, alliances, and ideologies. Broadly speaking, they each belong to one of four networks.

PRO-GOVERNMENT FORCES AND THEIR BACKERS

Syrian forces and pro-government militias

Syria’s armed forces and security services (mukhabarat) have long been a vital source of the Assad family’s control of Syria. But after suffering thousands of casualties and desertions, they have come to rely on the support of local irregulars and foreign militiamen, as well as Russian air power, to besiege and bombard opposition-held territory, recapture that territory, and restore control. With their support, Assad’s forces have turned the tide of the war. Since the fall of east Aleppo in December 2016, they control the country’s five most populous cities.

Alawis, members of Assad’s sect, make up much of the army’s top echelons; other minorities are also disproportionately represented among the officer corps. Sunni Arabs, who may comprise nearly two-thirds of the army, are also represented. Conscription is mandatory for all Syrians, and fighting-age Syrian men have been swept up at government checkpoints.

Assad has pledged to restore the state’s authority across all of Syria. But many analysts believe that the state exists in name only across many of the territories ostensibly under its control. Rather, they say, these territories are ruled as fiefs by local warlords. Pro-regime units are often seeking out more parochial interests, whether to earn black-market profits or defend local communities.

What are their capabilities?

The Syrian Arab Army may have as few as 25,000 troops that can be deployed to clear and hold territory. Its overall pre-war strength of some 220,000 active-duty troops has been diminished through casualties of conflict or defections. It has come to rely on militias of regime loyalists that range from groups of neighborhood thugs known as shabeeha (derived from the word “ghost”) to the more professional National Defense Forces, composed of military reservists.

The air force is perhaps the most potent wing of the military, with more than 260 aircraft plus attack helicopters, according to the International Institute for Strategic Studies (IISS), a London-based think tank. The regime has used air power to target not just rebel formations but also populated centers and civilian institutions protected under international law, including hospitals, often with crude barrel bombs.

Such air strikes, along with chemical weapons attacks, have drawn investigations into violations of international humanitarian law. Though the regime gave up some of its most lethal chemical weapons in 2013 amid threats of U.S. intervention, allegations of chlorine attacks persisted, and a sarin attack attributed to the regime in April 2017 prompted U.S. reprisal.

Who are they fighting?

The Syrian army and its allied militias are primarily fighting the Sunni-majority opposition, particularly to consolidate its control of the country’s main population and economic centers along its western spine. They mostly withdrew from Kurdish-majority areas in the north in 2011, and have largely ceded the country’s sparsely populated desert east to the Islamic State.

Hezbollah

The Lebanese Shia movement was established during the country’s civil war and expanded its support by putting up a guerrilla resistance to Israel in subsequent years. Its involvement in Syria’s civil war, though, has implicated the group in Assad’s killing of civilians and political repression, eroding its popularity in the Arab world. In Syria, it has galvanized a mostly Sunni opposition that now sees Hezbollah as a sectarian partisan and a beachhead of Iranian domination of the country.

Syria is a “lifeline” for Hezbollah, providing a pipeline for arms from Iran to Lebanon as well as areas to train. A hostile, Sunni-led successor regime could shut down that support; just as worrisome would be an anarchic Syria in which Sunni extremist groups could thrive. Thus, Hezbollah, at first in secret, answered Iran’s call in 2011 to put down the initial Syrian revolt. It claims to be a bulwark against Sunni extremist movements on the rise in Syria.

Though Hezbollah and Russian forces are tactical allies, efforts at a political settlement will expose differences between them, analysts say. Opposition forces will likely insist on Hezbollah leaving the country if Russia succeeds in brokering peace talks.

What are its capabilities?

Hezbollah has sent military advisors, and eventually, its elite forces and ground troops to fight in Syria. Its forces in Syria numbered between 4,000 and 8,000 at the start of 2016, says IISS, and, with Syria’s infantry weakened, they have been vital to rolling back opposition forces and holding territory cleared by Russian air strikes. It has besieged pro-opposition towns such as Madaya and Zabadani, northwest of Damascus near the Lebanese border, and oversaw their evacuations in what some analysts viewed as a project of demographic engineering to install Shia majorities in strategic areas.

Like its Syrian and Iranian allies, it has suffered from heavy battlefield losses, and estimates of fatalities range upwards from a thousand. Its chief military commander in Damascus was killed in May 2016 amid murky circumstances.

The group’s arms convoys have reportedly attracted Israeli air strikes. (Israel, which maintains it is neutral, has not confirmed them.)

Who is it fighting?

Hezbollah militants have primarily fought Sunni opposition forces, particularly along the southwestern border Syria shares with Lebanon. They were decisive in the 2013 battle for al-Qusayr, in which the opposition threatened to cut off a vital regime route between Damascus and the coast, as well as that year’s recapture of the Qalamoun mountains and town of Zabadani on the Syria-Lebanon border from rebel groups.

Foreign Shia militias

Iranian military advisors and Hezbollah militants have been reinforced by other Shia militants, primarily from Afghanistan, Iraq, and Pakistan. Some of the Iraqi militias are offshoots of the Popular Mobilization Fronts fighting to retake Iraqi territory captured by the Islamic State. The Afghan fighters are largely refugees who have long resided in Iran and were recruited by Iran’s Revolutionary Guard Corps with offers of citizenship or payoffs.

Their original aim was to defend Shia holy sites that they believe would be wiped out if Sunni militants toppled Assad’s government. Foremost among them is the tomb of Sayyida Zainab, the prophet Mohammed’s granddaughter, in a southern Damascus suburb. But as pro-Assad Syrian forces have been depleted by defections and casualties, these foreigners have come to a broader defense of the Assad regime, ranging across Syria’s frontlines against opposition groups.

What are their capabilities?

Their ranks have been estimated to be as high as 25,000. They have proven vital to the regime’s ground fighting, particularly in the battle for Aleppo.

Who are they fighting?

They have primarily clashed with Sunni-led opposition forces, particularly over the contested, populous western spine of the country. Among their battlefield foes are U.S.-backed opposition groups.

Iran

Syria is Iran’s main ally in the Arab world, and Tehran entered the conflict fearing that any successor to the Assad regime led by the country’s Sunni majority would align with its rival Saudi Arabia. As the civil war has dragged on, its fears have shifted to the threat of anarchy in Syria, which would foster conditions in which Sunni jihadi groups could thrive. It has focused most of its efforts in the country’s west, where opposition groups most directly threatened the regime.

In addition, Iran maintains its access to Hezbollah through Syria. It relies on Hezbollah as an asymmetrical deterrent to a militarily dominant Israel, and Syria offers a conduit to arm the Lebanese group. It is also pressing its strategy through demographic engineering, some observers say, including deals in which besieged pro-opposition populations have been evacuated to the northwestern province of Idlib, allowing pro-regime forces to take over strategically valuable areas and Shia majorities to settle there.

What are its capabilities?

Iran has helped keep the Assad regime economically afloat even while it was bearing the weight of international sanctions for its nuclear program. Early on it dispatched military advisors, and later, members of its elite Quds Force and Revolutionary Guard soldiers, their first major deployment abroad. They numbered up to two thousand at the start of 2016, according to IISS. Like Hezbollah, Iran had concealed the depth of its military involvement until mounting funerals made it apparent to the public. It is reported to have suffered several hundred casualties.

Russia

Moscow’s ties to Syria long predate the civil war, and it provided the Assad regime with a diplomatic shield at the United Nations after the start of the uprising. Then, in December 2015, it intervened militarily, focusing on supporting Assad’s campaign in the west and north, particularly in its bid to recapture rebel-held eastern Aleppo. Though Russia claimed to primarily be fighting the Islamic State, analysts said most of its operations have concentrated on other antigovernment groups and civilian targets in opposition-held population centers. The last rebel-held neighborhoods of Aleppo fell in December 2016 after a prolonged siege and intense bombardment. With the opposition weakened, Russia launched a diplomatic process in Astana, the Kazakh capital, that sought to instate a nationwide cease-fire as the groundwork for an eventual political settlement.

Russia’s interests include protecting its military bases on the Mediterranean coast. A deep-water port at Tartus offers Russian naval ships and nuclear submarines their only access to the eastern Mediterranean that does not require transiting the Bosporus, which is controlled by Turkey, a NATO member.

Russia wants to see the regime’s structure remain intact, and its officials cite the anarchy and civil war that followed NATO-led regime changes in Libya as reason to caution against Western support for antigovernment forces in Syria.

What are its capabilities?

Russia has deployed fighter jets and attack helicopters in population centers, providing government-aligned ground forces close air support to retake territory.

Activists and monitoring groups have accused Russia of bombarding such population centers as east Aleppo as part of a scorched-earth strategy meant to deplete rebels and encourage civilians to evacuate or capitulate. Allegations include the use of bunker-buster bombs, which wreak particular destruction on shelters and medical facilities built underground to withstand bombardment. France and the United Kingdom have accused it of war crimes; Moscow has denied targeting civilians.

OPPOSITION FORCES AND THEIR BACKERS

The Syrian opposition groups have sought to pose a national alternative to the regime. In most cases, though, they have been at best loosely coordinated with the local committees that sprang up to provide governance in areas wrested from regime control, as well as with the exile political groups that claimed to represent the opposition in international diplomacy.

These predominantly Sunni forces are fluid, with shifting memberships driven not just by groups’ stated ideologies but also by fluctuating support from external backers. Classifying groups as “moderates” or “extremists,” as the U.S. government does, is complicated, but they can roughly be divided by the political program they’ve espoused. At one end, the Free Syrian Army, the first major group to emerge, has advocated for a democratic and pluralistic Syria. At the most extreme are groups whose political program or governing style can hardly be distinguished from that of the Islamic State.

Free Syrian Army

The FSA was the main group to emerge when the regime first cracked down on protesters. Though it was led by defected army officers and comprised many former Syrian army conscripts, its name was something of a misnomer: With just scarce resources, its leadership could never centralize command and control over the many militias that had affiliated with it, and many of those groups began operating as criminal outfits. Today, opposition brigades in the south fly its flag, while in the north, brigades that identify with the FSA have largely been marginalized.

What are its capabilities?

The FSA received light arms, and later, artillery, including antiaircraft equipment, from its regional backers. It also received some nonlethal aid from the United States, but Washington has often been reluctant to arm even the groups it has vetted out of fear that heavy weaponry might fall into the hands of Islamist and jihadi groups

Who are they fighting?

FSA-aligned militias are the predominant opposition force in the country’s south. There, the loosely aligned Southern Front, with some thirty thousand fighters, has largely held territory captured from the regime with U.S. and Jordanian backing.

In much of the more fiercely contested north, by contrast, FSA fighters tend to be subordinate to jihadi groups with better resources. There they have primarily fought regime forces, but also clashed at times with the Islamic State and Kurdish People’s Protection Units (YPG)—sometimes alongside Turkish special forces.

Nationalist jihadis

Many Islamist militants reject the FSA’s advocacy of a democratic and pluralistic Syria and speak instead of remaking the state and society to adhere to fundamentalist principles of Islam. Unlike al-Qaida or the Islamic State, however, they do not claim to have transnational ambitions. Militants fighting under the FSA’s flag began to drift toward the jihadi camp in 2012, as the FSA suffered battlefield defeats and failed to muster resources and international backing.

This camp includes Jaish al-Islam, which is the predominant opposition group in the Damascus suburbs, and Ahrar al-Sham, which is active in the north and is seen by some analysts as sharing al-Qaida’s worldview.

What are their capabilities?

These groups, which are comprised of tens of thousands of fighters, are the most capable in the opposition, reportedly armed with tanks as well as anti-tank missiles and other artillery. They emerged on the Syrian battlefield in mid-2011, when Assad released from prison more than a thousand insurgents who had fought in Iraq after 2003. They soon eclipsed the FSA, attracting many of its fighters with resources provided by Qatar, Saudi Arabia, and Turkey.

Who are they fighting?

These groups, which are focused on fighting the regime, remain largely limited to territory in northern Idlib, Aleppo, and Hama provinces, as well as pockets around Homs and Damascus. They have often allied with the former al-Qaida affiliate Jabhat al-Nusra (now known as Jabhat Fatah al-Sham), while fighting it at other times. They have also clashed at times with FSA units, the Islamic State, and YPG.

Jabhat Fatah al-Sham (formerly Jabhat al-Nusra)

The Front for the Conquest of the Levant (JFS) was known as Jabhat al-Nusra until July 2016, when its leader, Abu Mohammed al-Julani, dissolved his relationship with al-Qaida. By claiming to have “no affiliation to any external entity”—and thus, no transnational ambitions—JFS hoped to court relationships with other opposition groups that were wary of it. But most observers doubt that the announcement marked a real shift in its allegiance or worldview. U.S. officials have expressed concerns JFS might welcome transnational terrorists in territory under its control, much as the Taliban had done in the 1990s in Afghanistan. The United States has targeted the group in air strikes and designated it a terrorist organization and successor of al-Qaida in Iraq, even as JFS is battling regime forces in some areas alongside U.S.-backed groups. Unlike the Islamic State, JFS principally comprises Syrians.

What are its capabilities?

JFS has proven to be effective fighting Assad’s forces. Many of its members gained experience fighting U.S. forces in Iraq during the insurgency against the U.S.-led occupation and sectarian civil war, and have used tactics from that insurgency, including suicide bombings on regime military installations. The group has benefited from official and private support from Gulf countries. One estimate puts its membership at upwards of 7,000.

Who is it fighting?

The Assad regime and its allies are its primary antagonists, and JFS has fought alongside a range of other opposition groups against pro-regime forces. It controls territory in northern Syria, particularly in Idlib province, near the Turkish border between Aleppo and Latakia. Its offensive in August 2016 briefly broke the regime’s siege of east Aleppo.

In 2013 it fought what was then known as the Islamic State of Iraq, reducing the group’s reach in northern Syria after the Islamic State’s Baghdadi had tried to claim Nusra as its subordinate.

United States

Just months into his presidency, Donald J. Trump dropped Barack Obama’s call for Assad to step aside. Within days, however, a chemical weapon attack on civilians prompted a U.S. reprisal. U.S. policy since then remains unclear: while Trump said the attack changed his view of Assad, the statement that accompanied the cruise-missile launch said it was meant to deter future chemical attacks.

Obama’s objective had been a negotiated transition that would have seen Assad give up power without jeopardizing Syria’s territorial integrity or the institutions of the state. Such an outcome, the administration said, would have alleviated the humanitarian situation and eased the burden on front-line states and U.S. allies in Europe posed by mass migration from Syria.

But since Russia has used its veto in the UN Security Council to protect Assad, the United States had shifted its focus from pressing for a negotiated settlement to carrying out a counterterrorism mission, while advocating de-escalation.

What are its capabilities?

The United States has used its air power narrowly for a counterterrorism mission, taking the lead in assembling a multinational coalition to counter the Islamic State, and has supported ground operations carried out by the Syrian Democratic Forces (SDF). It has unilaterally carried out a separate campaign against militants believed to be aligned with al-Qaida in northern Syria that escalated in the final months of the Obama administration and continued after Trump took office.

It has launched aircraft from the Incirlik Air Base in Turkey as well as a carrier group in the Mediterranean, and deployed troops to assist SDF ground forces. The Obama administration was reluctant to provide similar support to opposition forces focused on Assad; however, it trained and equipped some opposition forces, believing that increasing military pressure on Assad would have brought him to negotiate a settlement. It also aided separate majority-Arab forces that were focused on fighting the Islamic State. But it declined to send even vetted groups the artillery that some experts say could have challenged the regime’s monopoly on air power, and its largest commitments have been to the SDF, which Washington has seen as the most expedient ground force for rolling back the Islamic State. U.S. forces have deployed to Syria to support them. Though the number of troops operating in Syria is capped at 503, the actual number may be higher. The Pentagon says their mission is “training, advising, and assisting” local forces, but with the campaign to retake Raqqa gearing up, they are increasingly close to combat.

Turkey

Turkey believed that Assad would quickly be overthrown and was an early backer of the Syrian opposition in 2011. Though it remains the opposition’s main patron, Turkey’s primary interest lies elsewhere, in blocking the emergence of an autonomous Kurdish region in northern Syria. As Syrian Kurds have set back the Islamic State and Arab opposition forces on the battlefield, they have consolidated territory under their control. Turkey fears that this will inspire intensified national aspirations within its own Kurdish minority just across the border and provide domestic insurgents with a staging ground for attacks. The main Syrian Kurdish armed group, the YPG, has links to the Kurdistan Workers’ Party (PKK), whose war with the Turkish state resumed in the summer of 2015 amid a breakdown in peace talks.

Its secondary interests include pressing for a political transition in Damascus and defeating the Islamic State, which has carried out attacks inside Turkey. But it has been at odds with U.S. efforts to roll back the Islamic State because of Washington’s reliance on the SDF, in which the YPG is the predominant player. It has lobbied the United States to send its own proxies, Arab forces, to wrest the city of Raqqa from the Islamic State, but U.S. officials have dismissed this plan.

After shooting down a Russian aircraft that apparently entered Turkish airspace in late 2015, the two countries appeared poised for conflict. Instead, Turkey signed on to Russia’s efforts to broker a cease-fire and, eventually, a settlement in Astana, acting as the interlocutor for the opposition groups that took part.

What are its capabilities?

Turkey has established a de facto “safe zone,” deploying ground troops to an area between the border towns of Jarablus and Azaz in tandem with allied Syrian Arab and Turkmen armed groups. It has driven out Islamic State troops while preventing two Kurdish cantons from linking up to form a single, contiguous territory. Though Turkey declared the operation over in March 2017, its troops have not withdrawn.

President Recep Tayyip Erdogan gave permission in July 2015 for the United States to conduct flights from the Incirlik Air Base. It allowed the United States to fly missions over Syria from far shorter distances, becoming Turkey’s main bargaining chip with the United States. Turkey also deployed F16s of its own.

Erdogan has purged the military after a coup attempt in July 2016, including many officers billeted to NATO headquarters. Combined with the resumption of a counterinsurgency in southeastern Turkey, that could prove a distraction from its aims in Syria.

Arab Gulf states

The Arab Gulf states’ primary concern is what they see as Iranian expansionism in the Arab world. They seek Assad’s overthrow, but are less interested in a democratic order taking his regime’s place. A second-order concern is counterterrorism, particularly with respect to the Islamic State, which is thought to be responsible for several attacks inside Saudi Arabia.

What are their capabilities?

Saudi Arabia and Qatar have been among the opposition’s chief backers, funneling arms and funds to the FSA and jihadi groups. U.S. officials have urged Gulf countries to crack down on citizens’ funneling of private funds to extremist groups like JFS, calling Kuwait and Qatar “permissive jurisdictions” for their fundraising.

The Gulf states helped form the Army of Conquest, which includes JFS and promised better coordination among a range of opposition groups in northern Syria. They have channeled it heavy artillery such as anti-tank weapons.

ISLAMIC STATE

This successor of al-Qaida in Iraq seeks to erase the border between Iraq and Syria and establish a state of its own there. Its claim to have resurrected the caliphate attracted tens of thousands of foreign fighters from the Arab world and beyond. The Islamic State primarily comprises non-locals.

What are its capabilities?

The Islamic State’s capabilities (and revenues from commodities like oil, as well as taxation) have been tied to its territorial gains and losses. Since its peak in mid-2014, it has been rolled back to the point at which its leaders have begun preparing their adherents for the loss of the territorial caliphate. One research firm, IHS, estimates that the group lost nearly 30 percent of its territory between January 2015 and October 2016. Some 6 million Iraqis and Syrians still live in areas under its control.

In June 2016 congressional testimony, U.S. envoy Brett McGurk estimated that the group comprised some 18,000 to 22,000 fighters, down from a high of 33,000 in 2014.

Who is it fighting?

The Islamic State is most prominent in the country’s northeast and eastern desert provinces contiguous with Iraq’s Anbar province. Some of the Islamic State’s fiercest fighting has been against the Kurdish YPG in the north, as evidenced by the devastation wrought in Kobani, on the Turkish border. The Islamic State has also clashed with the erstwhile Jabhat al-Nusra and other opposition groups.

Some analysts say that in the uprising’s early days the Assad regime maintained a sort of nonaggression pact with the group. But as the group expanded, the regime took action against it, particularly around Palmyra and Deir ez-Zour. Opposition groups declared war on the Islamic State in January 2014 as reports of its atrocities against civilians mounted.

KURDISH PEOPLE’S PROTECTION UNITS

The YPG is the militia arm of the Democratic Union Party (PYD), the Kurdish party that de facto governs the Kurdish-majority cantons of northern Syria known as Rojava. The PYD says it seeks an autonomous Rojava within a decentralized Syria, though Turkey suspects it of harboring separatist ambitions. The PYD has ties to the PKK, which has waged a decades-long insurgency in Turkey and is designated a terrorist organization by the United States. It is the lead party in the U.S.-backed SDF, which also includes Arab militias.

What are its capabilities?

The United States considers the YPG the most militarily capable group confronting the Islamic State, which is why it has embedded military advisors with the SDF and provided it with air support to fight the militant group over the objections of its NATO ally Turkey. Its forces numbered some 50,000, the Syrian Observatory for Human Rights estimated in 2015.

Who is it fighting?

The YPG’s main foe has been the Islamic State, which it has fought for control in northern Syria, particularly along the Turkish border. It retook the cities of Kobane and Tal Abyad from the group in 2015, though at great cost, rolling it back from the Turkish border. Its main goal is to link up its cantons in the north in a contiguous territory, which has also brought it into conflict with Arab opposition groups.

U.S. support, however, has led it to stray far from Kurdish-majority areas. The United States has urged the SDF to take the lead in wresting the Sunni-majority city of Raqqa from the Islamic State, a venture that has raised the ire of many local Sunni Arabs as well as their patron, Turkey. Reportedly, an agreement struck between the United States and the SDF had Kurdish elements committing to refrain from liberating and occupying Arab-majority cities in this offensive.

Alexander Decina contributed. This backgrounder first appeared on April 28 on the Council on Foreign Relations’ website.