Captain Sieber was the pilot of a F-80C Shooting Star fighter interceptor with the 80th Fighter Bomber Squadron, 8th Fighter Bomber Group. On 4 September 1950, while on a combat mission of strafing enemy positions, his aircraft received a direct hit by anti-aircraft fire and crashed between Kimpo Air Base and Seoul, South Korea. He was listed as Missing in Action and was presumed dead on 31 December 1953. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

For his leadership and valor, Captain Sieber was awarded the Distinguished Flying Cross, the Air Medal with 6 Oak Leaf Clusters, the Purple Heart, the Korean Service Medal, the United Nations Service Medal, the National Defense Service Medal, the Korean Presidential Unit Citation and the Republic of Korea War Service Medal.