Alcatraz Island later invalidated the sale and paid Frémont nothing. Frémont and his heirs sued for compensation during protracted but unsuccessful legal battles that extended into the 1890s. Following the acquisition of California by the United States as a result of the Treaty of Guadalupe Hidalgo (1848) which ended the Mexican–American War, and the onset of the California Gold Rush the following year, the U.S. Army began studying the suitability of Alcatraz Island for the positioning of coastal batteries to protect the approaches to San Francisco Bay. In 1853, under the direction of Zealous B. Tower, the United States Army Corps of
Alcatraz Island self-determination, in part as a result of the publicity and awareness created by the occupation. The occupation ended on June 11, 1971. The entire Alcatraz Island was listed on the National Register of Historic Places in 1976, and was further declared a National Historic Landmark in 1986. In 1993, the National Park Service published a plan entitled "Alcatraz Development Concept and Environmental Assessment." This plan, approved in 1980, doubled the amount of Alcatraz accessible to the public to enable visitors to enjoy its scenery and bird, marine, and animal life. Today, American Indigenous groups, such as the International Indian Treaty