Remember the last time we decided to let the Russians do a significant share of the fighting for us in a war? What resulted from that? Oh, right, 44 years behind the Iron Curtain for Eastern Europe. Because the Russians never do anything unless they've first calculated how they're going to advance their own geopolitical interests as a result.
So when someone need to bomb the bejeezus out of ISIS and Barack Obama wasn't interested, why sure, Vladimir Putin would be only too glad to help: