Oil
Beat Assad
Defeat Assad
Save the Shia
Obey Iran
Protect the Kurds
Stabilize Lebanon
Save the Saudi's Bacon (They birthed ISIS, now they need to kill it before it comes for Mecca)
The missile and bomb makers paid for more business
To use as a carrot to get Iran to sign a nuke deal (OKOK, there is only at .000000000056% chance that the Obama administration is both this capable and this strategic.)
So that he can tell the people that he is doing something. He will hope that we dont notice that he has put us in a new dumb war.