Until recently the United States viewed the Islamic State in Iraq and Syria, or ISIS, as a major threat to regional stability in the Middle East. Barack Obama made it a mission to roll back ISIS’s territorial and propagandistic gain, and Donald Trump campaigned on a promise to “kick ISIS’s ass.” The United States expended considerable effort, both military and political, in a campaign to defeat the terror group in Iraq and, to a lesser extent, Syria. But there is also no doubt that the bulk of the effort came from Iran, not the United States. Without Iranian involvement, ISIS...