In 1942 during World War II, Claiborne was admitted to practice in his home state of Arkansas, but since the war had just begun, he enlisted in the United States Army and was sent to the Santa Ana Army Air Base. He served in the military police force. He would later say that he was inspired by his father's stand against the Ku Klux Klan and so refused to go along with the internment of Japanese Americans in camps, and was punished for doing so. Whatever happened, by 1943, Claiborne was promoted to warrant officer and transferred to what is now Nellis Air Force Base near Las Vegas, Nevada. His job was to investigate AWOL (absent without leave) incidents. Later, he guarded prisoners and supervised the military police who patrolled downtown Las Vegas. He was briefly stationed in New Mexico and then Massachusetts.