When we went into Iraq, we had three reasons: 1) It has weapons of mass destruction. 2) It has ties with terrorist organisations (Al Qaeda), and 3) It's a dictatorship. Well, the first 2 turned out to be wrong. But, at the exact same moment in time, all 3 were relevant to Pakistan. 1) It had proven nuclear weapons, 2) It had proven terrorist training camps, and links to Al Qaeda (Where did we find Osama?) and 3) It was under dictatorship.
So, why Iraq and not Pakistan?