The seizure of Mosul and continuing ISIS advance has now captured the attention in the United States that it deserves.
I still think that the U.S. should do nothing, with the possible exception of some limited air support. ISIS does not have the power to take Baghdad. Shia soldiers and militia will fight to defend “their” territory, and the Iraqi army is becoming increasingly sectarian anyway. Air support might help contain bloodshed by stopping an ISIS advance, but it is not clear to me that abetting an Iraqi counteroffensive is a good thing. The only reason to allow for an Iraqi counteroffensive is if we believe both that Maliki can extirpate ISIS and that the resulting deaths would be necessary for American security. I tend to doubt either proposition.
I am, however, open to counterarguments. My inner isolationist can be overwhelmed by my inner imperialist, given enough evidence.
ADDITION (three minutes later): The reason I do not believe that Maliki can extirpate ISIS is simply that he has proven remarkably uninterested in making the political accomodations necessary to gain Sunni acquiescence. He might be able to lead a genocidal campaign, but I doubt his army’s ability to carry that out ... and it is certainly not something that the United States wants to support.