In 1863, during the Civil War, the U.S. Army established a camp on the island (now known as Camp Reynolds or the West Garrison), and it subsequently became an infantry garrison during the US campaigns against Native American peoples in the West. In the later nineteenth century, the army designated the entire island as “Fort McDowell” and developed further facilities there, including what is now called the East Garrison or Camp McDowell. During the Spanish-American War the island served as a discharge depot for returning troops. It continued to serve as a transit station throughout the first half of the twentieth century, with troops engaged in the First and Second World Wars embarking and returning there.
During World War II, Japanese, and German POWs were also held on the island. The army abandoned the island in 1946, but returned in the 1950s when a Nike missile base was constructed; this was decommissioned as obsolete in 1962.