Earlier this week, Mosul—the largest city under Islamic State’s control—fell to Iraqi forces, while Islamic State has been driven almost entirely from its capital of Raqqa in Syria. Reports even circulated yesterday that the caliph, Abu Bakr al-Baghdadi, had been killed. With Islamic State (IS) poised to fall, Elliott Abrams explains what’s next for the U.S.

The defeat of Islamic State as a “state” will leave two serious questions facing the United States. The first is: who will fill the spaces from which the jihadist group is driven? There is a clear effort by the new Iran-Hizballah-Shiite militia-Russia coalition to reply: “We will.”

That is an answer the United States should reject. Such a development would cement an anti-American coalition in place, threaten Jordan and Israel, and leave Iran the dominant power in much of the region. To reject this challenge verbally would be a joke, however; it must be resisted on the ground, through the use of force by a coalition that must be built and led by the United States. . . .

[O]ne can envision a discussion with Russia of how our interests and theirs can be accommodated while bringing the violence down to a level that allows many refugees to return home. But that discussion will achieve nothing unless American power first gains Russian respect and the Russians come to realize that compromise is necessary.

Even in the best-case scenario, with IS defeated and losing its control over a “state,” it may continue to exist as a terrorist group—and in any event al-Qaeda and other jihadist groups will not disappear. . . . So long as Iran tries to dominate the entire region and Sunni jihadist groups target the United States, the defeat of the Islamic State changes—but does not diminish—America’s stake in Middle East power politics.

Two extraordinary events demonstrate something important about Israel’s most fervent adversaries. One was a speech given at something called The People’s Forum (funded generously by Goldman Sachs), which stated, “When the state of Israel is finally destroyed and erased from history, that will be the single most important blow we can give to destroying capitalism and imperialism.”

The suggestion that this tiny state is the linchpin of a global, centuries-old phenomenon like capitalism goes well beyond anything resembling rational criticism. Even if Israel were guilty of genocide, apartheid, and oppression—which of course it is not—it would not follow that its destruction would help end capitalism or imperialism.

The other was an anti-Israel protest that took place in front of New York City’s Memorial Sloan Kettering Cancer Center, deemed “complicit” in Israel’s evils. At organizers’ urging, participants shouted their slogans at kids in the cancer ward, who were watching from the windows. Given Hamas’s indifference toward the lives of Gazan children, such callousness toward non-Palestinian children from Hamas’s Western allies shouldn’t be surprising. The protest—like the abovementioned speech—deliberately conveyed the message that Israel is the ultimate evil and its destruction the ultimate good, cancer patients be damned.

The fact that Israel’s adversaries are almost comically perverse does not mean that they can be dismissed. If its allies fail to understand the obsessive and irrational hatred that it faces, they cannot effectively help it defend itself.