The Spanish presence in the territory of what is now the United States of America lasted three hundred years. The presidio-missions binomial was the cornerstone of its conquest, Christianization and defense, both from the internal threat posed by the Indians, and from the external threat posed by other powers. Presidio makes a broad review of the existing documentation on this important chapter of history, the conquest of North America, in those first and complicated decades, and does so with a simple narrative, in which the protagonists themselves take the floor to narrate the events.