Striking IS in Iraq is only half the answer. If it is driven out, its fighters will regroup in Syria only to hit back later. That is why IS must be clobbered in Syria as well as in Iraq.
Mr Obama should now do what he ought to have done all along: give a decent supply of arms to Syrias moderate rebels, who are equally menaced by IS. But even with weapons and money from outside, the moderates will not be able to dent IS. That needs aerial attack from America and its NATO allies.