Why was Alcatraz a military base?

Answer 1

The U.S Army first used Alcatraz as a military fortress between the 1850s and the 1900s. In 1915, the Army then renamed the island to the Western U.S. Military Prison U.S. Disciplinary Barracks and was used as a holding place for mischievous enlisted personnel. In 1933, the Army left the island and left it in control of the Federal Government for the use of the new Penitentiary.

Answer 2

Due to it's position in San Francisco Bay, Alcatraz was initially used a a fort to guard the harbor. In 1915, with the Navy now guarding the west coast, the Army converted it to a prison. However upkeep was expensive, and the Army gave it to the Federal Government in 1933 to be used as a federal prison.

Answer 3

in the past it was used for prision know it is used for tours