That may have been true in the beginning, but now Iraq is the epicenter of the war against Islamist extremist terrorism. We didn't make it that way, the terrorists could have just as easily moved in to Afghanistan. As far as justification, we actually had MORE justification to return to hostilities with Iraq, since Hussein constantly broke the Ceasefire of Safwan 1991.
Of course, any justification argument is debating history now. The facts on the ground are what matter now, and the fact is, us leaving won't mean that Syria, Iran and the Islamist Extremists will to.