After being bombarded by four years of U.S. airstrikes, a relentless ground campaign by Kurdish-led U.S. allies, and attacks from Russian, Syrian, and Iranian forces, the Islamic State has lost the last vestige of its physical caliphate.

But that defeat was neither final nor decisive, and policymakers should heed the War on Terror’s lessons to ensure the West doesn’t squander this advantage and enable ISIS, or its copycats and successor groups, to rally.