Fuerte de San Diego was built to defend Acapulco, the Pacific terminus of Spain's Manila galleon trade. Beginning in the sixteenth century, ships crossed from the Philippines carrying silk, porcelain, spices, lacquerware, and other Asian goods. Cargoes landed at Acapulco and traveled overland toward Mexico City and Veracruz, while American silver moved west to purchase Asian merchandise. The annual arrival of a galleon concentrated extraordinary value in a harbor exposed to pirates and wartime attack, making a permanent fort essential to the Spanish Crown's transpacific commercial system.

Viceroy Diego Fernández de Córdoba, Marquis of Guadalcázar, ordered the first major fort in 1615. Dutch engineer Adrián Boot designed an irregular five-sided work completed about 1617. Its bastions commanded the bay, protected warehouses and anchored ships, and provided refuge during alarms. The fort deterred raiders and symbolized royal control, but tropical weather, maintenance problems, and earthquakes repeatedly tested it. A powerful earthquake in 1776 caused such severe damage that authorities decided to replace rather than simply repair the older structure.

Engineer Ramón Panón designed the present regular pentagonal star fort. Construction began in 1778 and was completed in 1783. Officially named Fuerte de San Carlos for King Carlos III, it continued to be known locally as San Diego. Five projecting bastions allowed overlapping fields of fire, while thick masonry, a moat, magazines, barracks, and protected interior spaces supported a prolonged defense. The rebuilding came as Spain attempted to modernize imperial fortifications after global wars had exposed the vulnerability of colonial ports.

The fort's most important combat occurred during the Mexican War of Independence. José María Morelos captured Acapulco in 1813, but the royalist garrison under Pedro Antonio Vélez held San Diego for months. Insurgents surrounded and bombarded the position, turning the fort into one of the last centers of Spanish resistance on the southern Pacific coast. Hunger and isolation finally forced its surrender on 19 August 1813. Control of the harbor gave the independence movement a major symbolic victory, although war continued elsewhere for years.

San Diego remained a military installation after independence. It served Mexican forces during later civil conflicts and was used during the French Intervention and the Mexican Revolution. As its strategic function declined, government agencies debated new uses. In the twentieth century it passed among federal institutions and hosted cultural activities. The National Institute of Anthropology and History regained responsibility in 1980, and the Museo Histórico de Acapulco opened there in 1986.

The museum now interprets Indigenous cultures, Spanish expansion, piracy, the Manila galleons, independence, and the fort's architecture. Fuerte de San Diego thus connects three oceans and several centuries of conflict. It defended a trade route that linked Asia, the Americas, and Europe; endured earthquake and total reconstruction; resisted a defining independence siege; and eventually became the institution preserving Acapulco's history rather than a weapon controlling its harbor. The fort's later museum role reconnects military history with the maritime networks it once guarded. Exhibits on trade, navigation, and Asian contacts help explain why Acapulco mattered beyond New Spain: the harbor joined American silver, Pacific shipping, and global exchange long before modern tourism transformed the bay.