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
Declaration of Independence of the Mexican Empire of the Nation in Mexico City. The document is wide and high. On September 27, 1821, eleven years and eleven days after the Grito de Dolores, the Army of the Three Guarantees headed by Agustín de Iturbide entered Mexico City, concluding the Mexican War of Independence. On September 28, Iturbide installed the Provisional Governing Board, comprising 38 people. The board was chaired by Antonio Pérez Martínez y Robles, and Juan José Espinosa de los Monteros was secretary. The board immediately elected the five members of the Regency of the Empire. On October 13 of the same year, Ramón Gutiérrez del