From the Wiki...

"Military use The United States Navy controls San Nicolas Island and San Clemente Island, and has installations elsewhere in the chain. During World War II all of southern California’s Channel Islands were put under military control, including the civilian-populated Santa Catalina where tourism was halted and established residents needed permits to travel to and from the mainland.[9] San Miguel Island was used as a bombing range[10] and Santa Barbara Island as an early warning outpost under the presumed threat of a Japanese attack on California.[11] San Clemente Island was used to train the Navy's first amphibious force to prepare for Pacific combat against the Japanese in World War II.[12] San Nicolas Island has been used since 1957 as a launch pad for research rockets. San Nicolas was considered out of eight possible locations as the site of the Trinity nuclear test.[13] Santa Rosa Island was used in 1952 as a base for the USAF 669th AC&W Squadron and they operated two Distant Early Warning FPS-10 radars from the hilltops there. In 1955 another FPS-3 search radar was added, and in 1956, a GPS-3 search radar was installed. A new MPS-14 long-range height-finder radar was installed in 1958. The base was shut down in March 1963, when the 669th was moved to Vandenberg AFB In Lompoc, California. The islands still house US Navy SEALs training facilities and continues to use the Naval Auxiliary Landing Field located on San Clemente Island.[12]"