The Mexican Frontier, 1821–1846 published by other historians. This work was supplemented by additional primary materials which he discovered in the archives that allowed him to delve further into certain areas. "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
Decolonization of the Americas further wars were fought, such as the Paraguayan War and the War of the Pacific. A related process took place in what is now Mexico, Central America, and parts of North America between 1810 and 1821 with the Mexican War of Independence. Independence was achieved in 1821 by a coalition uniting under Agustín de Iturbide and the Army of the Three Guarantees. Unity was maintained for a short period under the First Mexican Empire, but within a decade the region fought against the United States over the border lands (losing bordering lands of California and Texas). Most of the heat