"What should the US do ?"

Since the U.S. had no rational business in invading Iraq in the first place, the U.S. should just stay the fvck out. Asking Iran to step in might actually be a rational action, they being neighbors and all, if not entirely friendly ones, and the U.S. having a track record of having done absolutely not one thing right with respect to either of those two nations in the entire history of our nation. Seriously, what makes anyone think that anything the U.S. does in response to this situation will turn out to be right?