What sort of catastrophe is he trying to avert? An ISIS march on Baghdad is one fear; so too is an ISIS infiltration of Jordan and, over time, of Saudi Arabia as well. (There is palpable worry among Middle East experts in the administration that Jordan, a resolute ally of the United States and an island of stability in a mainly unhinged region, could become vulnerable to an ISIS juggernaut). Another goal is to deliver public defeat to a group that appears to many young, disaffected Muslim men to be the strongest horse (to borrow from Osama bin Laden) in the Middle East, and to disrupt the movement of young Muslim men from Europe (and elsewhere) who seek to affiliate with ISIS, and who could bring ISIS terror back to their own countries. (It is worth noting that many of these thousands of young men hold citizenship in countries that participate in the U.S. visa-waiver program.)

The president has promised the eventual neutralization of ISIS, but this task will be exceedingly hard, and he obviously knows that this is the case. He does have a couple of things working in his favor. The first is that the American public is with him. The videotaped beheadings of two American journalists have galvanized allegedly war-weary Americans for a fight in a way that we haven’t seen in years. The second is that leaders of moderate Sunni states are so frightened by ISIS that they seem ready to join the coalition Obama is building to fight the group.

But there are a daunting number of impediments to success. The first is that American public opinion is fickle. American pilots will be flying risky missions, and there are already American boots on the ground—special-forces boots, intelligence boots, trainer boots. American opinion could change if soldiers start getting hurt or killed, especially months from now, when the horrific images of those beheadings fade from memory. In any case, Americans are notably uninterested in sustaining long campaigns against even atrocious enemies.

The second obstacle is that the U.S. has no effective allies on the ground. An air campaign can achieve a great deal, but the campaign Obama is envisioning—slow and steady, rather than shock and awe—has the disadvantage of giving ISIS time to dig in. The U.S. will be counting on a dysfunctional Iraqi army; intermittently effective Kurdish guerrillas; Sunni tribes of dubious loyalty; and, of course, the Free Syrian Army—the force that Obama has spent three years disparaging as a collection of farmers and carpenters and engineers—to do the hard work of rooting ISIS out of presumably fortified towns and cities. Much of what Obama is promising in this ramped-up campaign is help for these outfits, but the U.S. has spent a dozen years training the Iraqi army, with negligible results.

Obama faces many other difficulties in this campaign. One of them is the quality of (so-called) U.S. allies such as Qatar, which plays every side of this conflict; and Turkey, which won’t even seal the border crossings across which would-be European jihadists travel on their way to ISIS recruitment stations. (This is not even to mention some of America’s European allies, who have provided millions of dollars in ransom to the ISIS treasury in exchange for their kidnapped citizens.) Another problem: End games are nearly impossible to envision at the moment, either in politically combustible Iraq, or in a collapsed Syria. Put another way, it is easier see the slippery slope than the exit door. Then there is a problem that recurs time and again in the Middle East: Will U.S. escalation against ISIS serve as a recruiting tool for the group? What will the U.S. do if ISIS terrorists launch successful attacks against American targets, either in the Middle East, Europe, or even at home? Further escalation will surely follow, but to what end? Without effective allies on the ground, fighting this fight on behalf of the U.S. and the civilized world, wouldn’t the U.S. have to eventually insert its own troops?