The XXI Corps was a corps of the U.S. Army during World War II. It was constituted on 2 December 1943, and activated on 6 December 1943 at Camp Polk, Louisiana. XXI Corps fought for 116 days in the European Theater of Operations, starting in the Alsace, crossing into southern Germany, and swarming into Austria, with individual elements reaching into northern Italy. The corps was commanded in combat by Major General Frank W. Milburn as a subordinate unit of the Seventh U.S. Army.

Lieutenant General Frank William Milburn was a senior United States Army officer who served during World War II and the Korean War.