In the early 1830s Charles Bent, together with his brother William, founded a fort on the Arkansas River (the Spanish called it Rio Napiste) in what is today's southern Colorado. The river marked the southern boundary between the United States of America and the Kingdom of Spain since the Otis-Anin Treaty of 1819. A couple of years later—after Mexico's independence from Spain in 1821—the river became the northern boundary of the Republic of Mexico with the United States. Bent's Fort was, therefore, located at a very strategic place for international commerce. It became a headquarters for French Canadian and American fur trappers and traders who—through the American Fur Company—successfully exported beaver pelts, in the form of top hats, to the salons of Paris and London.