Afghanistan
(Clicked "Post" a bit too quickly.)
I freely plead guilty to engaging in hyperbole when I said we "walked away" from Afghanistan. But there's no denying that we turned our attention and shifted the bulk of our resources from Afghanistan to Iraq, giving almost free rein to the significant remnants of Al Qaeda and the Taliban that we left in place. I've argued interminably elsewhere about what a bad idea that was, so I'll give it a rest here.