A US-led coalition technically brings together some 60 nations.
But air strikes by countries other than the United States have been relatively few, and regional powers continue to support different rebel groups on the ground.
The Russians and Iranians, meanwhile, have maintained their firm support for Syrian President Bashar al-Assad, thus complicating efforts to reach a diplomatic solution.
The US has been bombing the Islamic State group in Iraq and Syria since mid-2015, accounting for the vast majority of the coalition's air strikes. But the aerial campaign has been relatively small, with around 10-15 sorties per day, compared with 250 sorties per day during NATO's Kosovo bombing campaign in 1999 and 110 per day in Afghanistan in 2001.
Having pulled troops out of Iraq after he came to power, President Barack Obama is unwilling to see large-scale ground forces return to the country. There are thousands of US advisors in Iraq, and last month the United States announced it had 50 members of the special forces in Syria. Analysts say many more covert agents are likely to be deployed there.
Iran is Assad's closest supporter, providing its own ground troops, who are supported by fighters from its Lebanese ally, Hezbollah. Having regained some of its international standing since the signing of a nuclear accord earlier this year, Iran has also been brought into negotiations on Syria's future since October. Analysts say that in strategic terms, it has gained the most from the unrest of the past decade, with the overthrow of Saddam Hussein in Iraq leading to a Shia-led government in that country.
Syria is Russia's last diplomatic and military foothold in the Middle East, and Moscow has been determined to maintain leverage by supporting Assad's regime, say observers. It began air strikes in September, saying it was targeting IS. But western nations backing moderate groups say the vast majority of its strikes have hit non-IS fighters who pose a more immediate threat to Assad.
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Bordering both Syria and Iraq, Turkey is a frontline state that analysts say has become both a major asset and a potential problem for NATO. President Recep Tayyip Erdogan wants the removal of Assad and has stated the country's opposition to IS, but he is also concerned that Kurdish rebels may use their successes in Syria and Iraq to demand greater autonomy in Turkish border regions. The shooting down of a Russian fighter jet that strayed into its territory has added another layer of complication.
Saudi Arabia and Qatar support rebel groups in Syria with a view to bringing down Assad and checking the growing power of Iran in the region, say experts.
France has launched a small number of air strikes in Iraq since joining the US-led coalition in 2014, and began bombing runs in Syria in September. It has been one of the staunchest voices calling for the removal of Assad, but its priority has now shifted to the destruction of IS after the Paris attacks of November 13.
ON THE GROUND
There are around 30,000 IS fighters in Syria, around half of them foreigners, according to French government estimates. There are some 80,000 fighters for other groups who vary in the extent of their extremism, including 10,000 who fight for Al Qaeda affiliate Al-Nusra.
The Kurds have won key victories against IS, notably in Kobane in June. The West hopes they will continue their advance, but experts say the Kurds are more interested in defending their territory and would be threatened if strong Iraqi and Syrian regimes re-emerged.
Different groups receive funding and arms from different outside actors, including the US, Saudi Arabia, Qatar and Turkey.
Jordan is drawing up a list of opposition delegations that will take part in peace talks.