"The Mexican Frontier" examines the southwestern United States, specifically the areas now known as Arizona, California, New Mexico, and Texas, in the twenty-five years between Mexico's independence from Spain and the beginning of the Mexican–American War. This resulted in the transfer of control of the border areas to the United States. In the early days of the Republic of Mexico, these areas comprised its northern border, including the territories of Alta California and Nuevo Mexico, and the province of Texas. The northern frontier developed separately from the Mexican interior, due in part to its distance from the capital in Mexico City and its relative proximity to the United States frontier and settlement by numerous English-speaking Americans. Its peoples also had to deal with dangers from increasingly hostile Indian tribes.