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.
Emperor
In Mexico, the First Mexican Empire was the first of two empires created. After the declaration of independence on September 15, 1821, it was the intention of the Mexican parliament to establish a commonwealth whereby the King of Spain, Ferdinand VII, would also be Emperor of Mexico, but in which both countries were to be governed by separate laws and with their own legislative offices. Should the king refuse the position, the law provided for a member of the House of Bourbon to accede to the Mexican throne.