When World War II began Landrum was Chief of Staff of the 3rd Infantry Division at Fort Lewis, and he received temporary promotion to colonel. In 1942 he was promoted to temporary brigadier general. Landrum commanded the 7th Infantry Division and other forces in the Aleutian Islands Campaign during combat as the United States reclaimed islands in the Aleutians which had been seized by the Japanese. For his service in Alaska Landrum received both the Army and Navy Distinguished Service Medals. Landrum was promoted to temporary major general in 1943 and commanded the 87th Infantry Division during its training in the United States. He went to Europe shortly before the D-Day invasion, with the U.S. First Army commander, Lieutenant General Omar Bradley, intending to keep him unassigned and available to command a division on short notice. The 90th Infantry Division took part in the D-Day invasion, coming ashore on D-Day plus one and conducting combat activities in Normandy. In June, 1944, Major General J. Lawton Collins, commanding the VII Corps, became concerned that the 90th Division was not performing effectively in combat. As a result, he relieved the division commander, Brigadier General Jay W. MacKelvie and two regimental commanders. Landrum was then assigned to replace MacKelvie. Landrum commanded the 90th Infantry Division during the rest of June and into August. During this period Landrum became involved in a verbal altercation with his assistant division commander, Brigadier General Samuel Tankersley Williams, and requested that Williams be reduced in rank to colonel and reassigned. Landrum’s superiors concurred, and the action was carried out. In August, Landrum’s corps, army and army group commanders were still not satisfied with the performance of the 90th Division, and Landrum was relieved. He was succeeded by Major General Raymond S. McLain, and returned to the United States. Upon returning to the United States Landrum succeeded Major General Robert L. Spragins as commander of the 71st Infantry Division during its mobilization and training before it departed for combat in Europe. He then commanded the Infantry Advanced Replacement Training Center at Camp Maxey.
Postwar
Following World War II Landrum returned to his permanent rank of colonel and continued his career, including assignment as Deputy Chief of Staff of Fifth Army in Chicago.
Korean War
During the Korean War Landrum served with Eighth Army, first as Deputy Chief of Staff and then Chief of Staff. At the time, commanders of numbered armies were not officially authorized deputies, so Landrum served as the de facto deputy for General Walton Walker, the Eighth Army commander. Walker, who had served with Landrum at Fifth Army, relied on him during organization of the Pusan Perimeter as Eighth Army organized the defensive action which enabled U.S. and South Korean forces to begin the counterattack that pushed the North Koreans north across the 38th Parallel. Walker, who always referred to Landrum as "General Landrum" in recognition of his temporary World War II rank, made Landrum responsible for finding replacement troops and reinforcements and employing them where they could be most effective. Landrum received a second award of the Distinguished Service Medal to recognize his efforts in Korea. Landrum served until reaching the mandatory retirement age of 60. He retired at his wartime rank of major general in a ceremony at Fort Mason, California.
Landrum was married to Frances Richardson Yeater, the daughter of Charles Yeater. Their children included son Eugene and daughter Marianna. Eugene M. Landrum, Jr. was a lieutenant colonel in the army and served during World War II and the Korean War.
