Syrian Proxy War Primer

http://www.zerohedge.com/news/2015-12-01/syrian-proxy-war-world-war-iii-esque-summary

The New York Times' attempt to explain who's fighting who: 
http://www.nytimes.com/interactive/2015/10/16/world/middleeast/untangling-the-overlapping-conflicts-in-the-syrian-war.html?_r=0

Why? Qatar needed to get its Qatar-Turkey pipeline through Syria, and Europe looked forward to linking up with the world's largest gas producer because it was over-dependent on Russian supplies. Russia had previously cut off gas supplies to Europe in the dead of winter 2009 after a dispute with Ukraine over gas royalties. Europe now urgently needs to find an alternate gas supply not controlled by Russia. This makes a middle-eastern pipeline coming through Syria a very attractive proposition.

The Assads soon realized that they were in a position of power. They decided to up the ante by creating an alternative source of fuel for a trans-Syrian pipeline. So instead of the Qatar-Turkey pipeline, Assad stitched together a deal with the Shia administration in Iraq, together with Iraq's other neighbor, Iran - the largest Shia nation in the world. The Iran-Iraq-Syria pipeline project was born.

http://oil-price.net/en/articles/oil-prices-and-syrian-civil-war.php