October 5, 1940, to April 4, 1941, when Fred L. Walker succeeded him. From April to September 1941 he was the first commander of the country's largest Infantry Replacement Training Center, Camp Wolters, located in Mineral Wells, Texas. He received a promotion to temporary major general on September 29, 1941, and commanded the 35th Infantry Division, an Army National Guard formation, at Camp Robinson, Arkansas, from October 15, 1941, to April 5, 1942, for which he was awarded the Legion of Merit. He then commanded the 30th Infantry Division, another Army National Guard formation, at Fort Jackson, South Carolina, until Leland Hobbs took command. On August 31, 1942, he took command of the newly created XII Corps. Simpson commanded the Fourth United States Army from September 29, 1943, to May 8, 1944, with the three-star rank of lieutenant general as of October 13, 1943. Fourth Army was re-formed when the combined headquarters of the Western Defense Command and Fourth Army were separated. A cadre was provided by the Western Defense Command, but all senior officers were approved or selected by Simpson. He brought his chief of staff, Colonel James E. Moore with him from XII Corps. Moore had previously served with him as chief of staff of the 35th and 30th Infantry Divisions. The Fourth Army headquarters was initially in San Jose, California, and it functioned as a training army. In search of more office space, the headquarters was moved to the Presidio of Monterey, California on November 1, and then to Fort Sam Houston in January 1944, when it took over the training mission of the Third United States Army, which had moved overseas. More personnel arrived in early 1944, enabling the Fourth Army to be split into a training army (the Fourth) and a headquarters to be deployed overseas, the Eighth, which was activated on May 5, 1944. Simpson and most of his staff became part of the Eighth Army headquarters. An advance party of the headquarters flew to the UK on May 11, and Simpson met with the commander of the European Theater of Operations, United States Army, General Dwight D. Eisenhower, an Army War College classmate. At Eisenhower's request, Simpson's command was redesignated the Ninth United States Army to avoid confusion with the British Eighth Army. The main body of Ninth Army embarked for the UK on the ocean liner on June 22. Simpson observed the