Eisenhower returned to the U.S. in December 1939 and was assigned as a battalion commander and regimental executive officer of the 15th Infantry at Fort Lewis, Washington. In March 1941 he was promoted to colonel and assigned as chief of staff of the newly activated IX Corps under Major General Kenyon Joyce. In June 1941, he was appointed Chief of Staff to General Walter Krueger, Commander of the 3rd Army, at Fort Sam Houston in San Antonio, Texas. After successfully participating in the Louisiana Maneuvers, he was promoted to brigadier general on October 3, 1941. Although his administrative abilities had been noticed, on the eve of the U.S. entry into World War II he had never held an active command above a battalion and was far from being considered by many as a potential commander of major operations.

What unit did Eisenhower serve with upon his return to the United States?