If there is one cause common to the governments of the U.S., Turkey, Iraq and Iran, it’s the continued suppression of Islamic State, the most destructive Islamist militant organization the world has seen. Working together and separately, those nations and their allies by early 2019 managed to subdue the group in Iraq and Syria, where it once controlled a chunk of territory as big as Iceland. Now, tensions among the countries that dismantled Islamic State threaten to subvert efforts to combat its