The Syrian civil war, now in its fifth year, involves multiple countries with overlapping agendas.
Competing visions of how to manage the conflict have led to a global refugee crisis as well as the rise of the Islamic State in Iraq and Syria (ISIS).
Here is where some of the main foreign actors stand.
United States: It backs more moderate elements among the rebel forces in Syria and opposes the government of President Bashar al-Assad, and ISIS and other Islamic extremist groups. It leads a coalition conducting air strikes in Iraq and Syria against ISIS and other extremist groups.
Russia: It supports Mr Assad who has been Russia's only persistent ally in the Middle East for decades. It has carried out its first attacks on ISIS targets within Syria and said it fears a blowback of thousands of young Russians who have joined ISIS coming home to carry out attacks.
Turkey: It is part of the US- backed coalition against ISIS and, tacitly, supports the rebel forces in Syria. It is principally against the Assad government and Kurdish groups allied with the PKK, an insurgent group active in Turkey. Turkey began air strikes and military incursions in July, mainly in northern Iraq against the PKK. It has also allowed the coalition to use its air bases.
Iran: An ally of Mr Assad, Iran has been providing military support, weapons and financial aid since the start of the civil war. But seizures of territory by Sunni insurgents and ISIS have weakened Syria's army, and there are signs that Iran is conserving its resources to defend government strongholds, including Damascus.
Saudi Arabia: It backs a number of rebel groups fighting the Syrian government. Its foreign minister warned that if a deal to remove Mr Assad was not reached, the shipment of weapons and other support to Syrian rebels would be increased. Saudi Arabia also began conducting air strikes against ISIS in Syria a year ago.
NEW YORK TIMES