When George W. Bush began talking about taking military action against Saddam Hussein, I thought it was not a good idea, but not for any of the reasons I typically hear bandied about by those who criticize the decision. Saddam was not a nice man, but he maintained stability in the region. By toppling him we would destabilize the region, and we would have to make a long-term, South Korea-esque commitment to keeping stability. We change leadership every four years, and I doubted the stick-to-it-iveness of subsequent administrations.
Every action has unintended and unanticipated consequences, and toppling Hussein was no exception. The unanticipated consequence was the complete collapse of the Iraqi government. With Hussein gone, the government just vanished and there was no centralized leadership to deal with—we were confronted with the unappealing prospect of trying to oversee the building of a new government out of chaos and anarchy. In many ways the situation we confronted was like the situation confronted by the Romans in Iberia after the Second Punic War. When the Romans destroyed the Carthaginian government in Iberia they created a power vacuum that it took 40 years and gallons of Roman blood to fill. The early Romans, however, never lacked stick-to-it-iveness and they ultimately prevailed. In Iraq we never intended to create a power vacuum, never expected to have to pay the bill in time and blood to fill it, and not even the Bush Administration was willing to voice a commitment to seeing the job through to the end.
The unintended consequence was the increasing militancy of Iran. Look at a map. Iraq is to the west of Iran, and Afghanistan is to the east. After the invasion of Iraq the U.S. Military occupied both countries. Put yourself in the shoes of the Iranian theocracy. America has taken down a dictatorship to your immediate west and a theocracy to your immediate east. Do you feel threatened? What are the two classic responses to threats? Flight or fight. Flight is impossible, so what is left? The Iranian government begins to  figuratively pound its chest and threaten the threatener (us) and  work to undermine our increasingly feeble efforts at reconstruction in Iraq and Afghanistan.
I didn’t see either of these consequences coming. All I foresaw was the need to invest years and years of effort into the reconstruction of Iraq and the basic unwillingness of subsequent administrations to make that investment. I, like everyone else, am better at hindsight than foresight. You don’t, however, need 20/20 hindsight to see that if we had maintained an adequate troop commitment to Iraq that ISIS would never have blossomed and grown.
Bush sowed the seeds of anarchy by invading Iraq, and Obama cultivated them by allowing the rise of ISIS. Bush’s mistake was to create a situation which required a commitment so lengthy and costly that subsequent administrations would be unwilling to follow through. We had a pretty good handle on things when Bush left office, but we would lose it if troops were withdrawn on the unrealistic timetable the Bush administration had set. Obama’s mistake was sticking to the unrealistic timetable. Somebody somewhere had to be telling him that if we did, things would go to Hell in short order. He had two options  stick to the unrealistic timetable and blame Bush for the resulting chaos, or  step up to the plate and stay the course. We have ISIS terror cells in the USA today because Obama took Option .