War in Syria: What countries are taking part and who are they fighting?

Syria’s bloody and intractable civil war became even more complicated this week when Russian warplanes started attacking rebel positions in support of the country’s embattled government.

What began in 2011 as a mostly peaceful uprising against Syrian President Bashar Assad has grown into a multi-sided war of attrition, in which alliances are constantly shifting, and an array of foreign powers are pursuing diverse and sometimes conflicting objectives.

Here are some of the main foreign players and who they are fighting.