        
Ironically, while built to guard against a foreign invasion, Alcatraz's most important period militarily was during the Civil War, 1861-1865. Since it There was some limited modernization of the island's defenses after the Civil War. Rifled cannons were mounted. In 1854 some 450 electrically controlled underwater mines were brought to the island to protect the Bay. However, as the ships of potential enemies became more and more powerful, the defenses were increasingly obsolete. In 1907 Alcatraz officially ceased being a fortress and became Pacific Branch, U.S. Military Prison.
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Fortress Alcatraz
San Francisco Bay is the largest natural harbor on America's West Coast. The gold rush of 1849 turned San Francisco from a sleepy village of 300 people into a booming city - and a tempting prize for possible foreign invaders. The Army's first plans were for forts on each side of the Golden Gate, with Alcatraz as a secondary defense. However Alcatraz became a primary fort almost immediately, when there were major obstacles to building a fort on the north side of the Golden Gate.
San Francisco's first defenses, eleven cannons, were mounted on Alcatraz in 1854. By the early 1860's Alcatraz had 111 cannons. Some were enormous, firing a fifteen-inch ball weighing 450 pounds. Defenses included a row of brick enclosed gun positions called casemates to protect the dock; a fortified gateway or Sally Port to block the entrance road; and a three-story citadel on top of the island. This served both as an armed barracks and as a last
line of defense.
was the only completed fort in the bay, it was vital in protecting San Francisco from Confederate raiders. Early in the war ten thousand rifles were moved to Alcatraz from the State armory, to prevent their being used by southern sympathizers, The crew of a Confederate privateer were among the island's first prisoners.
