From 1802 to 1813, Nemesio Salcedo served as commandant general of the Spanish Internal Provinces, making him the preeminent military and civil authority over a roughly 1.5 million square mile region that stretched from the western bank of the Mississippi River to the eastern border of modern California and south to what is today north-central Mexico. As commandant general, Salcedo's duty was to keep the peace with indigenous peoples, promote economic and population growth, and maintain Spanish sovereignty over western North America during the most tumultuous time in the history of western North America.Trouble began in 1803 when the Louisiana Purchase placed an ambitious and expansionist United States on the doorstep of the Internal Provinces. With inadequate resources Salcedo was forced to oppose what the Spanish regarded as illegal expeditions by the likes of Lewis and Clark and Zebulon Pike, led by American General James Wilkinson. Smugglers, escaped enslaved people, hostile Native American tribes, and even the subversive efforts of agents sent by Napoleon and former US Vice President Aaron Burr further destabilized the border and threatened Salcedo's rule.Amid crises, Salcedo pursued progressive initiatives that placed him among the most forward-thinking people of his day. He initiated a smallpox inoculation program that saved incalculable lives, established the first school for girls in western North America, and sponsored scientific expeditions meant to map, measure, and better understand the lands and peoples under Spanish claim.On a level with Thomas Jefferson, Miguel Hidalgo, and Napoleon Bonaparte, Nemesio Salcedo reshaped the geopolitics of the American west in the early nineteenth century. The Gatekeeper: Nemesio Salcedo and the North American West introduces readers to an important, yet heretofore less recognized leader during a period of profound change.