Mexican Independence was not an inevitable outcome, but events in Spain directly impacted the outbreak of the armed insurgency in 1810 and its course until 1821. Napoleon Bonaparte's invasion of Spain in 1808 touched off a crisis of legitimacy of crown rule, since he had placed his brother Joseph on the Spanish throne after forcing the abdication of the Spanish monarch Charles IV. In Spain and many of its overseas possessions, the local response was to set up juntas ruling in the name of the Bourbon monarchy. Delegates in Spain and overseas territories met in Cádiz, Spain, still under Spanish control, as the Cortes of Cádiz, and drafted the Spanish Constitution of 1812. That constitution sought to create a new governing framework in the absence of the legitimate Spanish monarch. It tried to accommodate the aspirations of American-born Spaniards (criollos) for more local control and equal standing with Peninsular-born Spaniards, known locally as peninsulares. This political process had far-reaching impacts in New Spain during the independence war and beyond. Pre-existing cultural, religious, and racial divides in Mexico played a major role in not only the development of the independence movement but also the development of the conflict as it progressed.

In September 1808, peninsular-born Spaniards in New Spain overthrew Viceroy José de Iturrigaray (1803–1808), who had been appointed before the French invasion. In 1810, American-born Spaniards in favor of independence began plotting an uprising against Spanish rule. It occurred when the parish priest of the village of Dolores, Miguel Hidalgo y Costilla, issued the Cry of Dolores on 16 September 1810. The Hidalgo revolt began the armed insurgency for independence, lasting until 1821. The colonial regime did not expect the size and duration of the insurgency, which spread from the Bajío region north of Mexico City to the Pacific and Gulf Coasts. After Napoleon's defeat, Ferdinand VII succeeded to the throne of the Spanish Empire in 1814 and promptly repudiated the constitution, and returned to absolutist rule. When Spanish liberals overthrew the autocratic rule of Ferdinand VII in 1820, conservatives in New Spain saw political independence as a way to maintain their position. Former royalists and old insurgents allied under the Plan of Iguala and forged the Army of the Three Guarantees. Within six months, the new army was in control of all but the ports of Veracruz and Acapulco. On September 27, 1821, Iturbide and the last viceroy, Juan O'Donojú signed the Treaty of Córdoba whereby Spain granted the demands. O'Donojú had been operating under instructions that had been issued months before the latest turn of events. Spain refused to formally recognize Mexico's independence and the situation became even more complicated by O'Donojú's death in October 1821.