The Washington Post has a lengthy look at how the U.S. military has balanced its efforts to combat ISIL with Iran-backed militias' parallel efforts.
"The expanding U.S. military campaign against the Islamic State group in Iraq relies in part on an uneasy, arms-length partnership with Shiite militias backed by Iran – organizations that were once relentlessly effective killers of U.S. troops."
"The Pentagon says it does not coordinate with Iranian-backed units. But since June 2014, when a lightning Islamic State advance pulled the United States back into military operations in Iraq, both sides have developed informal arrangements for avoiding conflict. For the United States, that has mainly meant carefully mapping locations of Iran-backed militias before launching airstrikes and making sure U.S. advisers keep their distance. In at least one case, that has put U.S. personnel and Iranian-backed operatives on opposite corners of the same base."