Tonight President Barack Obama will make his case to the country for a new and possibly far-reaching military entanglement in the Middle East -- a challenging task in any circumstances, but especially for Obama, because this isn't an argument he wanted to make. Up to now, the guiding theme of his security policy has been disentanglement.
So he has some explaining to do. In one way, his reluctant-warrior stance helps. Nobody can accuse him of rushing into battle. It also helps that Islamic State, the new threat, is an unusually clarifying kind of opponent: ambitious, organized, implacable and savage. There's not much ambiguity about Islamic State.