19. Entering into active use as a military fortress to protect the newly acquired Californian territories, the Alcatraz Citadel was completed and garrisoned in 1859
Decreed by President Millard Fillmore in 1850, following the end of the Mexican American War in 1848 and the acquisition of California under the Treaty of Guadalupe-Hidalgo, Alcatraz Island was set aside for military purposes. Starting in 1853, the United States Army Corps of Engineers began construction work fortifying the island with a budget of $87,689. Completing the Alcatraz Citadel, also known as Fort Alcatraz, in 1859, the structure took great advantage of the natural defenses of the island itself, using the steep rocky outcrops and rock faces to create an encircled and highly impenetrable fortress.
Entering into operation under the command of engineer James B. McPherson, the three-storied citadel included gun positions, roads, and even a moat and draw bridge. Enjoying the capacity to house a total of two hundred soldiers, with enough storage to outlast a siege of many months, in late-1859 the first soldiers were deployed at Alcatraz. Commanded by Captain Joseph Stewart, eighty-six men of Company H., Third U.S. Artillery, became the first garrison to inhabit the island, terminating their stay with the onset of the American Civil War two years later.