It might also have something to do with the survival of the "petro-dollar".
The petro-dollar is an agreement to buy oil from Saudi Arabia (or their oil alliance) with U.S. Federal Reserve notes, which is also considered the world reserve currency. It's an attempt to monopolize the purchase of oil from sources that will only take one kind of currency over another, and to protect that monopoly with force. It's a way to literally prop up the forced currency, which may well lose it's value if it wasn't needed to buy the damned oil from the monopoly.
That's what has started wars, and what might start this one.
Meanwhile, Iran will take other currency for it's oil, and Syria has said it can run a pipeline to the world through it's borders, and Russia supports this.
There is indeed a rival alliance, a monopoly, that want's to run through Syria to reach Europe.