California became a possession of the United States on February 2, 1848 in a treaty with Mexico that ended the Mexican War. A week earlier, on January 24th, gold had been discovered in the foothills of the Sierra Nevada Mountains. Within three years, the population of San Francisco would explode from around 500 to more than 35,000 as gold seekers poured into California.
Hundreds of ships, headed for San Francisco during the Gold Rush, wrecked along the dangerous California coastline. The first lighthouse on the West Coast of the United States was built on Alcatraz to guide ships safely into San Francisco Bay. The lighthouse went into service on June 1, 1854.
The U.S. Army, realizing San Francisco Bay was vulnerable to enemy attack, fortified the harbor entrance with strategic batteries including a fort on Alcatraz Island. The fort was completed in December 1859. During the American Civil War (1861–1865) Alcatraz became the largest American fort west of the Mississippi River.
The army began sending soldier-convicts to the Alcatraz fort in early 1860. Over the next forty years, the Island gradually became obsolete as a fortification and more important as a prison. The U.S. Army removed the fort’s guns and in 1907 formally designated Alcatraz as a Military Prison.
The army renamed the Island in 1915 as “Pacific Branch, U.S. Disciplinary Barracks”—a prison for soldiers undergoing punishment and retraining. Army prisoners built most of the buildings on the Island. This would be the final military role for the Island until the last soldiers departed in 1933.