MATTHEW CONTINETTI: The ISIS Endgame.

ISIS doesn’t have a chance. American air and ground forces, working with local proxies, are about to terminate its existence as a state. “Crushed,” to paraphrase President Trump. A just—and popular—cause.

But that won’t be the end. Recent events suggest that the military defeat of ISIS is just the beginning of a renewed American involvement in Iraq and Syria. And whether the American public and president are prepared for or willing to accept the probable costs of such involvement is unknown. That is reason for concern.

To glimpse the future, look at the city of Manbij in northeast Syria. Humvees and Strykers flying the American flag have appeared there in recent days. The mission? Not to defeat ISIS. Our proxies kicked them out last year. What we are doing in Manbij is something altogether different from a military assault: a “deterrence and reassurance” operation meant to dissuade rival factions from massacring one another. If you can’t remember when President Obama or President Trump called for such an operation, that’s because they never did.

And there’s a twist. One of the factions we are trying to intimidate is none other than the army of Turkey, a NATO member and purported ally. Turkey moved in on Manbij not because of ISIS but because of the Kurds. Recep Tayyip Erdogan, the Turkish autocrat, opposes one of our Kurdish proxies. He says the YPG is the Syrian affiliate of the Kurdistan Worker’s Party, which has conducted an insurgency against his government for decades. Yet the YPG is also the most effective indigenous anti-ISIS force on the ground. We need it to take Raqqa.

Things get even more complicated.

In the Middle East, things always do.