Yeah, it was so stable and peaceful up until then.
He didn’t say he blamed the chaos on the Iraq war. He said he blamed the (current) Iraq war on the chaos there. You seem to have read that backwards.
As easy as it is to want to blame Obama, the truth is it was Bush who set the withdrawal timetable, and the elected government of Iraq didn’t want to extend it. The Shiites couldn’t wait to boot the Sunnis out of any power in the government, which is why the Sunni troops don’t consider fighting ISIS their battle. Both sides wanted us out so they could get back to killing each other.
Ironically, Joe Biden may have been right about something for the only time in his life when he suggested the only long term solution might be splitting Iraq into Sunni, Shiite and Kurdish states. OTOH, as long as Muslims are killing each other, we can hope they will be too busy to cause us trouble.