Twenty C-17 Globemaster IIIs participate in a Large Formation Exercise Dec. 21 over the coast of Charleston, S.C. The C-17s, assigned to the 437th and 315th Airlift Wings at Charleston AFB, were part of the largest formation in history from a single base and demonstrated the strategic airdrop capability of the Air Force. (U.S. Air Force photo/Master Sgt. Andy Dunaway)

In this Thursday, Feb. 23, 2017 photo, U.S. Army soldiers stand outside their armored vehicle on a joint base with Iraqi army south of Mosul, Iraq. As Iraqi forces push into western Mosul coalition troops are closer to frontline fighting than ever before. Coalition forces have moved their bases closer to the front, relaxed their rules of engagement and during the push on Mosul airport coalition advisors were embedded with forward Iraqi rapid response and special forces units. Coalition officials say the change is helping speed up Iraqi military gains, but it marks a steady escalation of U.S. involvement in Iraq that could undermine the sustainability of Iraq's territorial victories. (AP Photo/ Khalid Mohammed)