The Syrian opposition and forces loyal to President Bashar Assad are not the only groups fighting in the conflict. Other countries have also intervened to pursue their own interests.
What it's done: Tehran has been one of Assad's strongest backers, supporting loyalist forces with money, weapons and intelligence. Iran has also sent military advisers from its Islamic Revolutionary Guard Corps to Syria and directed fighters from Hezbollah, a Lebanon-based militant group backed by Iran, which is also involved in the conflict. It has also organized paramilitary militia from Syria, Afghanistan and Iraq to fight for the Assad regime. Iran has set up multiple bases in Syria, raising concern in Israel that Tehran plans to stay in Syria for the longhaul.
Why it's there: Iran and Syria had a mutual defense pact before the onset of the Syria civil war in 2011. Iran has sought to bolster the Assad regime in its fight against various rebel factions, many of which are backed Tehran's regional rivals such as Saudi Arabia and the Gulf States. Iran, which also backs Shiite militia in Iraq fighting the "Islamic State," views the Syrian war as a frontline against Sunni militant groups and as a means to expand its regional influence. The Assad regime allows Iranian aid to flow to Hezbollah, a major regional military power and enemy of Israel.
What it's done: Moscow came to Assad's aid in late 2015, deploying hundreds of troops and acting as the regime's air force. Russian officials say it targets terrorist organizations like "Islamic State" (IS). But Russian bombers have also struck other anti-Assad groups, turning the tide of the war in the regime's favor. As a veto-wielding UN Security Council member, Russia has also provided the Syrian regime diplomatic cover. Alongside Iran and Turkey, Moscow has sought to find a political end to the conflict parallel to UN talks in Geneva.
Why it's there: Moscow wants to secure its influence in the Middle East by keeping Assad in office and maintaining an important military air base in the western province of Latakia and a naval base in the port city of Tartus. Russian President Vladimir Putin also appears to want to bolster Russian prestige and influence in the Middle East at the expense of the United States, which it blames for creating instability.
What it's done: Riyadh has given money and weapons to Syrian opposition forces, including some Islamist militant groups such as the Army of Islam. It has also played a limited role in US-led international coalition against IS.
What it's done: Turkey had a good relationship with Syria in the mid-2000s. Since the outbreak of the civil war, Turkey has supported non-Kurdish Syrian opposition groups seeking to topple Assad. Turkey has allowed opposition fighters, including jihadist militants, to enter the fray across the Turkish-Syrian border. In addition to fighting IS, the Turkish military and its rebel allies have conducted two operations in northern Syria against the Kurds and carved out a zone of influence.
Why it's there: Turkey originally sought to topple the Assad regime by backing various rebel groups. After Russia intervened in Syria in 2015, Turkey has focused more on preventing Syrian Kurds from gaining autonomy in northern Syria. Ankara fears that Kurdish gains could embolden the Kurdistan Workers' Party (PKK), which has fought a more than three decade insurgency against Turkey. Ankara considers the US-backed Syrian Kurds as a terrorist group tied to the PKK. Some 3 million Syrian refugees are in Turkey and Ankara seeks to prevent new refugee flows and carve out safe zones in northern Syria to house refugees.
What it's done: Israel has launched airstrikes against the Lebanese Shiite militia Hezbollah and Iranian targets in Syria, especially suspected weapons shipments and bases. It has also backed several smaller rebel groups and Druze along the border in the Golan Heights to create a buffer zone.
Why it's there:
Israel wants to prevent Iran from developing a political and military presence on its northern doorstep. Israel also wants to rollback Hezbollah in Syria in order to prevent it from forming a wider northern front, alongside its main backer Iran.
Hezbollah and Israel last fought a war in 2006, but since then the group has become stronger. Israeli intelligence estimates Hezbollah has more 100,000 missiles stockpiled, including advance missiles provided by Iran, that in the event of a war with Israel would overrun its missile defense system and be able to strike cities as far as southern Israel.
Why it's there: Washington's foremost stated goal has been the destruction of IS and other extremist groups in Syria. US policy toward Assad is less clear. Trump's predecessor, Barack Obama, said "Assad must go." Apart from its opposition to the Syrian government's use of chemical weapons, the Trump administration's position on Assad's future is more ambiguous. The US also seeks to rollback Iranian influence in Syria.
What it's done: Germany has flown surveillance flights over Syrian territory to support airstrikes against IS. Berlin has also called on Russia and Iran to persuade Assad to leave office in any peace deal. Germany is a major funder of humanitarian aid to Syrian refugees.
Why it's there: Berlin also wants to see the defeat of IS, which has carried out terror attacks in Europe. It has also opposed the Assad regime. German officials have said there can be no lasting peace in Syria if Assad remains in power.
What it's done: France initially sent medical supplies and weapons to opposition forces. In 2015, it began airstrikes against IS that intensified after an IS terror attack in Paris in November 2015. French special forces are deployed on the ground alongside US special forces backing the SDF. Paris has also warned Assad against using chemical weapons.
Why it's there: Paris wants to defeat IS after a string of IS-related terrorist attacks in France. French President Emmanuel Macron said in 2017 his country would no longer condition peace talks on a promise by Assad to leave office.