The Armada of the Strait under Don Diego Flores de Valdés in 1581–84 came at a crucial juncture in global politics. Philip II of Spain had assumed the crown of Portugal and its overseas empire, and Francis Drake’s daring peacetime raids had challenged the dominance of Spain and Portugal in the Americas. The armada was intended to ensure the loyalty of Portuguese Brazil; bolster its defences against hostile native peoples, and English and French pirates and interlopers; and fortify and settle the Strait of Magellan to prevent further incursions into the Pacific. Pedro de Rada, the official scribe of the armada, kept a detailed, neutral chronicle of the venture which remained in private hands until 1999 but is now held in the Henry E. Huntington Library in San Marino, California. It is published here for the first time. Previous historical assessments of the expedition have largely reflected the writings of Pedro Sarmiento de Gamboa, governor-designate for the planned colony at the Strait, who blamed all the misfortunes of the enterprise on Diego Flores de Valdés. Rada’s Relación is presented here in conjunction with other documentation and compared with Sarmiento de Gamboa’s accusations. The results will force scholars to revise long-standing conclusions regarding the place of Sarmiento and Flores in Spanish history and the accomplishments of a long-forgotten armada sent into the terrifying waters of the South Atlantic.