The first is that not all of ISIS’s enemies are terrible: The defense of the Kurds and of Jordan are causes worth pursuing. (Even the defense of Saudi Arabia is in American national security and economic interests.) An unimpeded ISIS threatens the Kurds, who are our allies and who have built for themselves something decent in their corner of Iraq, and it poses an existential threat to Jordan. If Jordan were to be overrun, this would spark both a humanitarian catastrophe of unimaginable proportions as well as a regional war between Israel (which would rush to Jordan’s defense) and the global jihadist movement, a war Iran would exploit to further its own anti-Israel and anti-Sunni objectives. ISIS infiltration of Saudi Arabia would be similarly disastrous. Even an ISIS move on Baghdad would be disastrous for the U.S.—imagine the mechanics of evacuating thousands of Americans from a city under ISIS siege. If you liked the fall of Saigon, you’ll love the pictures from besieged Baghdad.

It would, of course, be lovely if the non-Iran-sponsored ground forces arrayed against ISIS were formidable. (Obama, as I’ve noted, has spent three years disparaging the fighting skills of the secularish Free Syrian Army, which has now become a linchpin of the American-led effort against ISIS.) But they are not. The only possible way to slow ISIS’s progress, and to possibly reverse it in some more-than-negligible way, is to provide air cover and intelligence and logistics support to our hapless allies on the ground.

A second reason: President Obama was careful not to speak of an imminent or specific ISIS threat to Americans, because none currently exists. But it is not implausible to argue that a Qaeda-inspired group of limitless cruelty and formidable financial resources, one that has an omnibus loathing for “infidels,” and one that has thousands of members who hold passports from countries that participate in the U.S. visa-waiver program, poses a non-trivial threat to American civilians. Disrupting ISIS by attacking its leaders in their Syrian safe havens, rather than simply attacking their underlings inside Iraq, seems justifiable.

David is right to argue that the U.S. is functionally aligning itself with Assad, Iran, and Hezbollah, and this is a terrible thing. Other critics of the president’s plan are right to point out its flaws and limitations, and to ask whether our anger at the beheadings of two American journalists is blurring our vision. (It’s not the worst thing, at times like these, to have a president who leans in the direction of reluctance.) But the question for David is, what are the consequences for American national security of continued ISIS success?