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
Mexico obtained independence from Spain and the Spanish Empire with the Treaty of Córdoba in 1821. It briefly experimented with monarchy, but became a republic in 1824. This government was characterized by instability, leaving it ill-prepared for international conflict when war broke out only two decades later, in 1846. In the decades preceding the war, Native American raids in Mexico's sparsely settled north prompted the Mexican government to sponsor migration from the United States to the Mexican province of Texas to create a buffer. However, the newly named "Texians" revolted against the Mexican government of President/dictator Antonio López de Santa Anna, who had usurped the Mexican Constitution of 1824, in the subsequent 1836 Texas Revolution, creating a republic not recognized by Mexico, which still claimed it as part of its national territory. In 1845, the Texan Republic agreed to an offer of annexation by the U.S. Congress and became the 28th state in the Union on December 29 that year.