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 would still have a formidable presence in both Iraq and Syria.
ISIS was born out of the ashes of the American invasion of Iraq. Their rise was the logical extension of a process that saw the fabric of secular Sunni society torn asunder by an American occupier unwilling to further empower a Sunni ruling elite that had been loyal to Saddam Hussein. Washington failed to understand the resentment engendered within the Sunni community when Iraq's Shia, some of whom were beholden to Iran, came to power.