Angel Island, largest island in San Francisco Bay, W Calif. Explored by the Spanish in 1775, it came under U.S. control in 1851. The U.S. army used the island as a base from 1863 to 1946, and from 1955 to 1962 a radar and missile site was there. From 1910 to 1940 the island was also used to process mainly Asian immigrants to the United States, earning it the nickname "Ellis Island of the West." During World War II, enemy prisoners of war were confined on Angel Island. The island is now a state park.