Syria's conflict is often called a civil war, but the reality is it spills across the country's borders and involves some of the most powerful countries in the world.
So who is backing President Bashar al-Assad and his government and who believes peace can only be achieved by the removal of his regime?
Supporting the Syrian government
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Russia
That Assad has survived while presiding over six years of civil war in Syria is due in large part, says CNN, to the military, diplomatic and economic support provided by Russia. Syria has for decades been one of Moscow's strongest allies in the Middle East.
Russian President Vladimir Putin's apparently unwavering support for his Syrian counterpart has had the effect of boosting Russia's reputation as an international power and Putin's standing as a leader to be reckoned with.
Iran
The Syrian conflict is, among other things, a proxy war through which the regional Shia-Sunni Muslim rivalry between Iran and Saudi Arabia has been played out. The fact that a high-ranking Iranian politician four years ago referred to Syria as "the 35th province of Iran" reflects its strategic importance for Tehran.
Although it denies the presence of its combat troops in Syria, Iran has provided arms, military advisers and financial support to the Assad regime. Meanwhile, Lebanon's Tehran-backed Hezbollah movement has sent thousands of fighters to Syria.
Opposing the Syrian government
The US
The United States was consistently critical of the behaviour of the Assad regime under Barack Obama but did not take any direct military action. US air strikes in Syria from 2014 were aimed at Islamic State and not the regime.
Days before the Idlib chemical weapon attack, Secretary of State Rex Tillerson said the "longer-term status of President Assad will be decided by the Syrian people" - but Donald Trump's administration may now be reviewing its policy on the country.
Turkey
Turkey is a crucial regional component in the conflict, sharing a long and porous border with Syria through which thousands of fighters and hundreds of thousands of refugees have flowed.
Turkish support for rebel groups fighting in Syria is complicated by the presence of the Kurdish People's Protection Units, the YPG. The Kurds, like Turkey, have been battling IS, but Ankara is worried that Syrian Kurds could establish a contiguous autonomous region along its border, strengthening the position of Kurds inside Turkey itself.
Saudi Arabia
Alongside other Gulf states and Jordan, Saudi Arabia has provided money and weapons to insurgent groups fighting the Syrian regime and Islamic State. The Saudi position, says the BBC, is that Assad cannot be part of a solution to the conflict and must hand over power to a transition administration or be removed by force.
