Back in 1991 after Saddam invaded Kuwait, I remember thinking "why the hell do we have to solve the problem? Why not force Saudi Arabia and the other Gulf States to take responsibility for their security?"
Practically all the Islamist terrorism that the US has experienced since then has flowed from that decision.
The likelihood of terrorism should be part of any decision making process when it comes to the Mideast, no different than estimating potential body counts from land invasion.
At least Eric Shinseki wasn't afraid to estimate the real costs of the Iraq war.