Mexican War of Independence
Iturbide's assignment to the Oaxaca expedition coincided with a successful military coup in Spain against the monarchy of Ferdinand VII. The coup leaders, part of an expeditionary force assembled to suppress the independence movements in the Americas, had turned against the monarchy. They compelled the reluctant Ferdinand to reinstate the liberal Spanish Constitution of 1812 that created a constitutional monarchy. When news of the liberal charter reached Mexico, Iturbide perceived it both as a threat to the status quo and a catalyst to rouse the criollos to gain control of Mexico. Independence was achieved when conservative Royalist forces in the colonies chose to rise up against the liberal regime in Spain; it was an about-face compared to their previous opposition to the peasant insurgency. After an initial clash with Guerrero's forces, Iturbide assumed command of the royal army. At Iguala, he allied his formerly royalist force with Guerrero’s radical insurgents to discuss the renewed struggle for independence.
Mexican–American War
The Mexican Army emerged from the war of independence (1810–1821) as a weak and divided force. Before the war with the United States, the military faced both internal and foreign challenges. The Spanish still occupied the coastal fortress of San Juan de Ulúa, and Spain did not recognize Mexico's independence, so that the new nation was at risk for invasion. In 1829, the Spanish attempted to reconquer their former colony and Antonio López de Santa Anna became a national hero defending the homeland. The army had a set of privileges ("fueros"), established in the colonial era, that gave it jurisdiction over many aspects of its affairs. In general, the military supported conservative positions, advocating for a strong central government and upholding privileges of the military and the Catholic Church.