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.
American frontier
Mexico became independent of Spain in 1821, and took over Spain's northern possessions stretching from Texas to California. Caravans began delivering goods to Mexico's Santa Fe along the Santa Fe Trail, over the journey which took 48 days from Kansas City, Missouri (then known as Westport). Santa Fe was also the trailhead for the "El Camino Real" (the King's Highway), a trade route which carried American manufactured goods southward deep into Mexico and returned silver, furs, and mules northward (not to be confused with another "Camino Real" which connected the missions in California). A branch also ran eastward near the Gulf (also called the Old San Antonio Road). Santa Fe connected to California via the Old Spanish Trail.