U.S. Marines and U.S. Airmen work together during a simulated contamination control area in preparation of Vigilant Ace 17-1 at Osan Air Base, Korea, Nov. 17, 2016. Vigilant Ace is a biannual, bilateral training event that exercises U.S. military and Republic of Korea Air Force interoperability through simulated wartime tasking. - U.S. Marines and U.S. Airmen work together during a simulated contamination control area in preparation of Vigilant Ace 17-1 at Osan Air Base, Korea, Nov. 17, 2016. Vigilant Ace is a biannual, bilateral training event that exercises U.S. military and Republic of Korea Air Force interoperability through simulated wartime tasking.