During World War II, he was the Chief of War Plans Division of the war Department General Staff. He would later be reprimanded for his actions in the lead up to the Japanese attack on Pearl Harbor. He commanded the 29th Infantry Division and then the V Corps. As such, Gerow played a major part in the planning of Operation Overlord, the invasion of continental Europe. He was the first corps commander ashore on D-Day, June 6, 1944, and continued in command through the Battle of Normandy, which saw his divisions sustain heavy casualties. He became the first American major general to enter Paris after its liberation. In January 1945, he assumed command of the Fifteenth Army.