Dupuy joined the Supreme Headquarters Allied Expeditionary Force in 1943 as news chief and acting director of public relations, reporting to supreme commander Dwight D. Eisenhower. He played a key part in publicizing the Allied landings on D-Day, the first day of the invasion of Normandy, when he had the role of officially announcing the operation on radio. In a broadcast at 12:32 am Pacific Time (3:32 am Eastern War Time) on June 6, 1944 he announced that "Under the command of General Eisenhower, Allied naval forces, supported by strong air forces, began landing Allied armies this morning on the northern coast of France". This announcement, known as Communiqué Number One, was so short that Dupuy read it out twice in a row.