Mexican War of Independence third phase.
Under Iturbide’s Plan of Iguala, the disputing factions of the Mexican citizenry were brought together, inspired by the piety expressed in support of Catholicism and the acceptance of a monarch (which the church taught was a divine right established by God). Iturbide dispatched messengers and emissaries to rebellious factions all over Mexico, explaining his position and soliciting their support. The Spanish forces and their allies, subject to constant guerrilla raids in the smaller towns and hamlets, began to withdraw towards Mexico City. Roving bands of guerrillas and more organized militia under the juntas rallied to Iturbide’s army.
In the summer of 1821 Iturbide’s army was too strong for the Spanish to overcome, and Iturbide dictated the Treaty of Cordoba, accepting the resignation of the Spanish viceroy, and establishing the independence of the Mexican Empire. Iturbide included a statement in the treaty which would allow the Mexican congress to appoint a criollo as Emperor should a suitable European prince not be persuaded to accept the throne. The Spanish Cortes did not accept the treaty, and in May 1822, after having his former regiments demonstrate in the streets of Mexico City demanding it, Iturbide was offered and accepted the throne and became Emperor Augustine I. In October the Emperor dismissed the Congress.
Spain attempted to reconquer the lost territory of what had been New Spain, through the ports of Veracruz and Ulua, and the Mexican Empire acquired warships from the United States and England, along with experienced naval officers to command them. Following the dissolution of the congress by the Emperor, the garrison at Veracuz, led by Antonio Lopez de Santa Anna, arose in rebellion against Iturbide, and gained the support of the Mexican fleet. The Imperial troops sided with Santa Anna, as did many of the heroes of Mexican independence and their followers. They demanded the restoration of Congress in the Plan of Casa Mata.
The Plan of Casa Mata was rapidly disseminated throughout all of Mexico, given extra urgency by the threat of the return of the Spanish. By the late winter of 1822 support for the plan was prevalent throughout all of Mexico save Mexico City, which was dominated by the Emperor and the few troops he still personally commanded. Face with the overwhelming strength of the opposition, which demanded a republican government, Iturbide reconvened the Congress, abdicated his throne, and ran for the hills, in March 1823.
The ensuing congress from the Mexican provinces and states (at the time the Mexican Empire stretched from California and as far north as present day Colorado to what is now Panama) established the Constitution of 1824, which established the United Mexican States as a federal republic. It prohibited any religion other than Roman Catholicism, created an independent judiciary, and though it did not specifically prohibit slavery, it did not repeal previously enacted laws which abolished slavery. In order to prevent Spanish incursion into the sparsely settled State of Texas, Mexico opened it to settlement from United States, offering citizenship and land to American settlers.