Since June, a great deal of international focus has been on Iraq, where the transnational jihadist movement Islamic State took over large swaths of the country’s Sunni-majority areas and declared the re-establishment of the caliphate. Despite the global attention on the country, especially given U.S. military operations against the Islamic State, U.S.-Iranian cooperation against the jihadist group — a significant dynamic — has gone largely unnoticed. A convergence of interests, particularly concerning the Iraqi central and Kurdish regional governments, has made it necessary for Washington and Tehran to at least coordinate their actions. However, mistrust and domestic opposition will continue hampering this cooperation.