Born in Indiana to an impoverished family, Shoup joined the military for financial reasons. Rising through the ranks in the interwar era, he was twice deployed to China during the Chinese Civil War. He served in Iceland at the beginning of U.S. involvement in World War II, and as a staff officer during the Pacific War. He was unexpectedly given command of the 2nd Marines, and led the initial invasion of Tarawa, for which he was awarded the Medal of Honor and the British Distinguished Service Order. He served in the Marianas campaign, and later became a high-level military logistics officer.