The four-year long war in Syria took another twist this week when Russia launched air strikes against targets in the country.
Here's what you need to know about this latest development in the increasingly bloody, complicated and volatile conflict.
Why have Russia launched air strikes?
Russian President Vladimir Putin says air strikes are a "preemptive" measure in a bid to stop Islamist jihadists advancing to his country.
Who are they supporting?
Russia is a staunch supporter of the Bashar al Assad regime, having previously sent weapons and military advisers to help the beleaguered president's fight against so-called Islamic State and Western-backed rebel groups.
Where have the bombs hit?
More than 30 strikes have already been conducted since the Russian Parliament unanimously back Putin's request to use military force overseas.
Multiple Islamic State targets - including ammunition and command centres - have been hit.
But most controversially, the Russians are also reportedly targeting Western-backed rebel groups.
US officials say Syrian rebels - some of whom have been trained by the CIA - have been targeted.
What have the rest of the world made of Russia's actions?
Put it simply: everyone is very, very nervous.
While welcoming any action to eradicate Islamic State, the US, Britain and other allies are concerned President Putin's motive is solely to keep Assad in power and extend Russia's influence in the region.
ITV News Washington Correspondent Robert Moore said the Americans were "scrambling to understand" the Kremlin's motives, tactics and targets.
Who else is involved? And who backs who?
The United States has taken the lead on air strikes against IS targets in Syria, launching its first attacks more than a year ago.
Despite intelligence reports suggesting the multi-billion dollar military campaign has had little effect on the strength of IS, the Obama administration is adamant that it will commit no US ground troops to the fight.
In a recent meeting between the US and Russian presidents, Barack Obama told Vladimir Putin that he saw no path to stability in Syria if Assad remains in power.
Britain's only military action in Syria during the current conflict has been the controversial RAF drone strike that killed Britons Reyaad Khan and Ruhul Amin near the Syrian city of Raqqa.
Prime Minister David Cameron has said there will not be stability in Syria unless President Assad is removed from power.
There is also a growing expectation that Parliament will debate and vote on the idea of joining air strikes in Syria against Islamic State targets.
Iran is widely considered to be Syria's closest ally, and on Thursday it was reported that hundreds of Iranian troops had arrived in Syria to join a major ground offensive in support of President Assad's government.
Last week, France launched its first air strikes against Islamic State targets in Syria.
Foreign Minister Laurent Fabius has said his country is ready to co-operate with Russia and others in the fight against IS in Syria, but said this was conditional on the removal of President Assad.
Jordan has been taking part in the US-led coalition air strikes against Islamic State militants in Syria.
Hundreds of thousands of refugees have fled to Jordan from Syria during the conflict.
Saudi leaders have long called for action against Assad and the country has been part of the coalition conducting air strikes against IS in Syria.
So what next?
US and Russian officials are holding talks about how they can both continue to carry out air strikes in Syria without getting in each other's way.
Russian foreign minister Sergey Lavrov has said his country and the coalition are working towards the same goal, and both the Russians and Americans have pledged to continue their bombing campaigns.
Suspicions will remain that Russia is only becoming more involved in Syria as a way of attacking moderate rebels and propping up Assad. This could polarise the battlefield between ISIS and Assad, forcing the West to back the Syrian regime as the only bulwark against a terrorist takeover of Damascus.
How exactly things will pan out remains to be seen.