Most people agree that in retrospect, the problem with the Iraq War was a lack of Phase IV (post-invasion) planning. Don't you think it might have been best to consider how to deal with Iranian-sponsored insurgent violence before actually invading? And what was at stake?

This is what the administration failed to do - they actually believed the situation on the ground would be stable after the invasion. And that was just dumb.

Ah, I see what you mean. Yeah, planning for the future wasn't one of the Bush Administration's high points. But they're guilty of mismanaging a lot of things other than the Iraq and Afghan wars, so that's nothing new.