@izzythepush,
By definition, you can't eliminate the impact of intervening causes through the original action that sets events in motion. No matter how poor the Bush decisions or how poorly he and his Administration acted, they couldn't possibly have made future decisions or actions better or worse.
We agree, though, that the post invasion segment of the strategy was poorly planned and executed.
We don't agree that it led to sectarian strife.
The Shiite resentment of the Sunni pre-dated the invasion. Toppling the regime allowed it to morph into action.
No carefully crafted plan for a post-war Iraq, no matter how well executed was going to erase sectarian strife, but it might have better controlled it.
The real culprit is Nouri al-Maliki, he more than Bush, Obama or anyone other than the leaders of ISIS is responsible for this mess.
Not sure, though, what the US could have been expected to do about Maiki. He won an election generally considered fair.
Saddam, had he not been overthrown, might possibly have been able to keep sectarian violence under wraps with the lash, but, in large measure, due to his oppression of the Shiia, it was simmering there all along; waiting to be unleashed.
The US under Bush and Obama all along urged the Iraqi government to be inclusive of the Sunni and the Kurds. The Kurds really just needed to be left alone, the Sunni needed to be "forgiven" and included. Clearly there was never a meaningful reconciliation effort made and Maliki has exacerbated the tensions, which opened the door for ISIS.
Still, despite, whatever one thinks about the decision to invade, and all the mistakes the Bush Administration made post-invasion, the Surge and counter-terrorism plan of General Petraeus stabilized the country. Obama may have been against the war from the beginning, but like the economy, he inherited Iraq. The condition of an inheritance is not an excuse for making it worse, and if it was a mess when passed along there was a responsibility to try and make it better.
While an argument can be made that a continued military presence in Iraq would have helped to curb the excesses of Maliki, no one can do so with convincing certainty that it would have. The resulting Sunni resentment and unrest might have set the stage for ISIS incursions, but a relatively modest but robust American military presence would have almost certainly been able to turn them back. The reported number of ISIS soldiers is only roughly 5,000.
The main issue under consideration now is what the US can and must do in response to the situation. If the consideration and decision can be helped by revisiting past decisions, then Obama's are as fair game as those of Bush, and since he, not Bush, is the current president, the onus is on him to make the right call.
Should we be fortunate enough to have a Republican president succeed Obama, I can't wait to see the response of his or her critics to defenses based on no more that "But Obama did the same thing! or "It was Obama's fault!"