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
Mexican real ¼, ½, 1, 2, 4 and 8 reales, with gold coins for ½, 1, 2, 4 and 8 escudos. Between 1814 and 1821, copper coins were also issued for ⅛, ¼ and tlaco. During the Mexican War of Independence (1810-1821), numerous mints operated, providing coins for both the supporters and opponents of the Spanish crown. The Royalist issued coins at mints in Chihuahua, Durango, Guadalajara, Guanajuato, Nueva Viscaya, Oaxaca, Real del Catorce, San Fernando de Bexar, San Luis Potosí, Sombrerete, Valladolid Michoacán and Zacatecas. Most Royalist issues were similar in style to the earlier colonial issues from the Mexico City