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 War of Independence of Iguala, Iturbide persuaded Guerrero to join his forces in support of this conservative independence movement. A new army, the Army of the Three Guarantees, was placed under Iturbide's command to enforce the Plan of Iguala. The plan was so broadly based that it pleased both patriots and loyalists. The goal of independence and the protection of Roman Catholicism brought together all factions. Iturbide's army was joined by rebel forces from all over Mexico. When the rebels' victory became certain, the Viceroy resigned. On August 24, 1821, representatives of the Spanish crown and Iturbide signed the Treaty of Córdoba, which