Reverting to his permanent rank of captain after the war, he taught military science at the University of Michigan. He transferred to the field artillery and was promoted to major on graduation from the Field Artillery School (1921), where he taught for two years. He graduated from the Command and General Staff School and then taught military science at Colorado Agricultural College (1924–1929). Following a command slot at Fort Bliss, he graduated from the Army War College (1932). He then served on the War Department General Staff. Promoted to lieutenant colonel (1935), he commanded the 1st Field Artillery Regiment at Fort Bragg, North Carolina (1936–1937). He next served on the Field Artillery Board (1937–1940) and was promoted to brigadier general (1940).
After briefly leading the 2nd Infantry Division, Lucas assumed command of the 3rd Infantry Division (July 1941) and was advanced to major general (August 1941). He then commanded III Corps in Georgia (1942–1943). Sent to the Mediterranean in mid-1943, he served as observer and deputy for Lieutenant General Dwight D. Eisenhower. Following the Allied conquest of Sicily, Eisenhower selected Lucas to command VI Corps in Lieutenant General Mark Clark's Fifth Army, although Lucas was not Clark's choice. Lucas's corps was Fifth Army's principal U.S. Army formation during its slow northward advance from Naples to the German Gustav Line at Cassino.
On 22 January 1944, Lucas commanded an uncontested amphibious landing at Anzio behind the German lines. Mounted with insufficient manpower and logistical support, the effort was undertaken in response to insistent pressure from Prime Minister Winston L. S. Churchill and the theater land forces commander, General Sir Harold Alexander, who anticipated that it would compel a precipitous German retreat from the formidable Gustav Line and open the way to Rome. Instead of striking immediately to the interior and securing the Alban Hills, Lucas consolidated the beachhead. Field Marshal Albert Kesselring rushed reserves there, and the Germans staged a powerful counterattack. Alexander blamed Lucas for the resultant stalemate, describing the cautious, pessimistic American general as "an old woman." Lucas was relieved on 22 February 1944, replaced by Major General Lucian K. Truscott.
Lucas headed the Fourth Army in Texas (1944–1946) and the Army Advisory Group in China (June 1946–January 1948) and was then deputy commander of Fifth Army in Chicago, where he died on 24 December 1949.
Richard G. Stone
Blumenson, Martin. Anzio: The Gamble That Failed. Philadelphia: Lippincott, 1963.; Clark, Mark W. Calculated Risk. New York: Harper and Brothers, 1950.; D'Este, Carlo. Fatal Decision: Anzio and the Battle for Rome. New York: Harper Collins, 1991.; Fisher, Ernest F., Jr. United States Army in World War II: The Mediterranean Theater of Operations—Cassino to the Alps. Washington, DC: Office of the Chief of Military History, Department of the Army, 1977.; Truscott, L. K., Jr. Command Missions: A Personal Story. New York: E. P. Dutton, 1954.