After completing fourteen home-study Army Extension Courses, Reagan enlisted in the Army Enlisted Reserve[32] on April 29, 1937, as a private assigned to Troop B, 322nd Cavalry at Des Moines, Iowa.[33] He was commissioned a second lieutenant in the Officers Reserve Corps of the cavalry on May 25, 1937.[34]
Reagan was ordered to active duty for the first time on April 18, 1942. Due to his nearsightedness, he was classified for limited service only, which excluded him from serving overseas.[35] His first assignment was at the San Francisco Port of Embarkation at Fort Mason, California, as a liaison officer of the Port and Transportation Office.[36] Upon the approval of the Army Air Force (AAF), he applied for a transfer from the cavalry to the AAF on May 15, 1942, and was assigned to AAF Public Relations and subsequently to the First Motion Picture Unit (officially, the "18th Army Air Force Base Unit") in Culver City, California.[36] On January 14, 1943, he was promoted to first lieutenant and was sent to the Provisional Task Force Show Unit of This Is The Army at Burbank, California.[36] He returned to the First Motion Picture Unit after completing this duty and was promoted to captain on July 22, 1943.[33]
In January 1944, Reagan was ordered to temporary duty in New York City to participate in the opening of the Sixth War Loan Drive. He was re-assigned to the First Motion Picture Unit on November 14, 1944, where he remained until the end of World War II.[33] He was recommended for promotion to major on February 2, 1945, but this recommendation was disapproved on July 17 of that year.[37] While with the First Motion Picture Unit in 1945, he was indirectly involved in discovering actress Marilyn Monroe.[38] He returned to Fort MacArthur, California, where he was separated from active duty on December 9, 1945.[37] By the end of the war, his units had produced some 400 training films for the AAF.[33]
Reagan never left the United States during the war, though he kept a film reel, obtained while in the service, depicting the liberation of Auschwitz, as he believed that someday doubts would arise as to whether the Holocaust had occurred.[39] It has been alleged that he was overheard telling Israeli foreign minister Yitzhak Shamir in 1983 that he had filmed that footage himself and helped liberate Auschwitz,[39][40] though this purported conversation was disputed by Secretary of State George Shultz.[41]