An emerging humanitarian catastrophe in Northern Iraq sparked U.S. intervention Thursday night. Islamic State, the radical group (previously called ISIS) that has been seizing territory across Northern Iraq, recently overwhelmed the Kurdish peshmerga forces defending Sinjar, a city home to hundreds of thousands of Yazidi—a religious minority considered by the militants to be devil-worshippers. In the last few days, around 200,000 Yazidis have fled Sinjar. Around 40,000 took desperate flight into the mountains, where they are now surrounded by heavily armed Islamic State fighters. Trapped in the desert, they have begun to die of thirst—including an unknown, but surely growing, number of children. Last night, the President announced that the United States was dropping food and water to the Yazidis stranded on the mountaintop. This morning, news broke that the U.S. had also bombed Islamic State artillery positions just outside the Kurdish capital of Erbil, where the radical fighters advanced late this week.

How did Islamic State become so fearsome? Joshua Keating notes the quiet role of Qatar and Saudi Arabia, which have unwittingly contributed to the group’s rise through their support of the Syrian rebellion (they’re among the opposition to Bashar al-Assad). Concerned about blowback, and subject to arm-twisting from the United States, these countries have lately tried to be more discerning about the recipients of their backing, but that’s much easier said than done. “For the last few months,” Keating notes, “the Saudi government in particular has been attempting, somewhat awkwardly, to both continue to fund non-extremist groups fighting Assad while combating the growth of al Qaeda and its affiliates and offshoots.”

As a result, Islamic State is now very heavily armed—which is how they overwhelmed the Kurdish militia defending Sinjar, how they’ve advanced so close to the Kurdish capital, and how they’re limiting access to the mountains where the 40,000 Yazidis are trapped. In the Washington Post yesterday, before the United States announced relief measures, Robert Farley pointed out that even simply dropping food and supplies to the surrounded civilians will be extremely difficult—not just because of the rugged terrain, but because “the ability of Iraqi or U.S. aircraft to operate safely is also in question. The Islamic State has demonstrated the capability to shoot down aircraft, and if the Yazidis are closely hemmed in, then relief aircraft could prove easy targets for Islamic State missiles.” Even the delivery of non-military aid will require air support—which Obama made clear when announcing the relief measures Thursday night.

To make matters even worse, on Thursday Islamic State captured Iraq’s largest dam, located just outside the city of Mosul. With control over that dam, as George Packer writes, the group could put Baghdad under as much as 15 feet of water; Mosul could be put under nearly 100 feet.

This alarming progression across Iraq raised the spectre of imminent slaughter—with Iraqis dying of thirst in the mountains and drowning in the cities. Earlier this week, Packer expressed skepticism that anything would be done:

It’s hard to know what, if anything, is left of the humanitarian responsibilities of the international community. The age of intervention is over, killed in large part by the Iraq war. But justifiable skepticism about the use of military force seems also to have killed off the impulse to show solidarity with the helpless victims of atrocities in faraway places.

Last week, in a piece on Libya, I critiqued the tendency of some writers to move the goalposts on humanitarian interventions: If interventions prevent or stop slaughter on a massive scale, they’ve fulfilled their (admittedly minimal) goals. The post-intervention humanitarian situation may still be deeply troubling, even if it’s not catastrophic, and it’s unrealistic to expect airstrikes (or even prolonged occupations) to solve that problem by establishing an effective government. Iraq is, of course, the obvious proof of that. Even interventions that succeed in preventing mass killings may nevertheless create power vacuums and lead to dangerous instability.

It’s important to recognize, however, that this is not an argument against interventions in every case. As always, we face a question of alternatives, and considered in that light, some interventions have succeeded (either by preventing an imminent slaughter or stopping an ongoing one). Yes, there are deeper political problems in Iraq that American airstrikes will not solve. The absence of a functional Iraqi government is a much longer-term issue, and the extent of American involvement in addressing that problem is a question of legitimate debate. But Islamic State is on the verge of being able to flood whole cities. They’ve trapped thousands of Yazidis on a desert mountaintop without food or water. Before condemning this policy choice, it’s worth grappling seriously with the alternatives.

Note to readers: I’ll be on vacation over the next week and a half and will return August 20th.