If we want to be historically accurate (ha! Here on HROT?) - the actual answer is European colonial powers (and to a lesser extent, the rapidly ascending-in-power U.S.) after WWI.
The re-drawing of the Middle East map in the aftermath of WWI has been an unmitigated disaster whose negative effects we continue to feel today. Particular blame needs to go to Great Britain, which carved up Palestine and Iraq with no regard to ethinic and religious boundaries that had been in place for hundreds of years.
But it's a lot more fun, I guess, to try and pin-the-blame on current and recent leaders - even though they've had the misfortune of having to play out an awful, losing hand dealt to them nearly 100 years ago.
That's great and all, but enough with the history lesson already....do you blame Bush or Obama?