1916 and later to captain on 15 May 1917, shortly after the American entry into World War I on 6 April 1917. From 16 January 1918 to 30 June 1920 Gerow served on the Signal Corps staff on the Western Front. He participated in the Second Battle of the Marne, Battle of Saint-Mihiel and the Meuse–Argonne offensive. He was promoted to the temporary rank of lieutenant colonel, in charge of purchasing all the radio equipment for the American Expeditionary Forces (AEF) in Belgium and France. For his services during the war he was awarded the Army Distinguished Service Medal and the French Legion of Honour. The citation for his Army DSM reads: After returning to the United States, he was promoted to the permanent rank of major on 1 July 1920. He attended the advanced course at the Infantry School at Fort Benning, Georgia, in the fall of 1924. He graduated first in the class in 1925 from the Advanced Course at the Infantry School. Omar Bradley graduated second. Gerow attended the Command and General Staff School, where Dwight D. Eisenhower was his study partner. In 1931 he completed the Field Officer's Course in Chemical Warfare and Tanks, and took a course at Army War College. His first wife, Kathryn Getchell, died on June 17, 1933. He then married to Mary Louise Kennedy, who died on October 29, 1970. He had no children. Gerow served in China in 1932 in the Shanghai sector during the January 28 incident. On 1 August 1935 he was promoted to the permanent rank of lieutenant colonel. On 1 September 1940, prior to the American entry into World War II, he became a colonel in the permanent grade and a month later, on 1 October 1940, he became a temporary brigadier general. World War II At the time of the attack on Pearl Harbor and the entry of the United States into World War II in December 1941, Gerow was Chief of War Plans Division of the war Department General Staff. He was promoted to temporary major general on 14 February 1942. On handing over the position of the Chief of War Plans Division to Dwight Eisenhower, Gerow told him: "Well, I got Pearl Harbor on the book; lost the PI [Philippine Islands], Sumatra and all the NEI north of the barrier. Let's see what you can do." In October 1942, Gerow became Commanding General