Marines with Weapons Company, 3rd Battalion, 7th Marine Regiment, fire mortars from an undisclosed location in Syria on Sept. 10, 2018. The United States’ main ally in Syria on Thursday, Dec. 20, 2018, rejected President Donald Trump’s claim that Islamic State militants have been defeated and warned that the withdrawal of American troops would lead to a resurgence of the extremist group. (U.S. Marine Corps photo by Cpl. Gabino Perez via AP)