Originally built as a fort to defend San Francisco Bay during the Gold Rush era, Alcatraz Island's isolation and treacherous currents made it an ideal site for confining military prisoners, including Confederate sympathizers and Native American captives. The island's reputation as an inescapable prison would culminate decades later in its transformation into a federal penitentiary.
Alcatraz's rocky isolation in the middle of San Francisco Bay, surrounded by frigid, current-swept waters, made it a natural chokepoint the Army had already fortified with cannons during the Gold Rush to guard against a naval attack that never came. Officers soon realized the same isolation made it a superb prison.
In its military prison years, the island held a strange mixture of inmates: American soldiers convicted of crimes, Confederate privateers and sympathizers captured during the Civil War, and Native American prisoners, including Hopi men from Arizona who had resisted federal orders to send their children to government boarding schools.
The island's reputation for total inescapability—built on genuine physical isolation rather than architecture—would eventually make it the natural choice when the federal government sought a maximum-security penitentiary in 1934, ultimately housing Al Capone and George 'Machine Gun' Kelly, and cementing 'The Rock' in American popular imagination.