Point form of how this mess go so bad:
Saudi (US Ally) wanted to run pipe line to sell natural gas to europe through Syria
Russia currently supplies most natural gas to western europe.
Syria declined due to their alliance to Russia who supplies them with weapons & equipment.
USA funded FSA - free syrian army (rebels) to oust Al Assad from Syria. FSA is funded & trained by CIA, but falls apart as many defect & form ISIS.
ISIS starts slaughtering, no one cares, until it looks like they might actually overthrow Al Assad so Russia steps in.
Russia does more damage to ISIS in 6 weeks than the US does in 6 months because the US doesn't really want eliminate them. They want Al Assad out of power so they & the Saudis can sell natural gas to western europe.
Basically, this is all about money & power via Natural Gas. Millions of people slaughtered or displaced because of greed. Nothing more.