The royalist brigadier, Agustín de Iturbide, proclaimed the independence of Mexico on 24 February 1821 at Iguala, a small town in the heart of the southern, tropical tierra caliente or ‘hot country’. In his manifesto, the Plan of Iguala, Iturbide called for independence, the union of Mexicans and Spaniards and respect for the Roman Catholic Church. The form of government was to be a constitutional monarchy in which the emperor would be chosen from a European, preferably Spanish, dynasty ‘so as to give us a monarch already made and save us from fatal acts of ambition’, and the national constitution was to be drawn up by a congress. With this the first of his so-called ‘three guarantees’, Iturbide won the support of the old guerrilla fighters for independence, particularly General Vicente Guerrero who at this time was operating not far from Iguala. The second guarantee offered security to Spanish-born residents of Mexico, and with the third he sought to attract the clerical establishment by promising to preserve ecclesiastical privileges, recently under attack in Spain by the liberal, revolutionary regime. The army would take upon itself the task of ‘protecting’ the guarantees.

Iturbide's appeal proved remarkably successful. In less than six months, he was master of the country, except for the capital city and the ports of Acapulco and Veracruz. It was at Veracruz that the newly appointed Spanish captain-general, Juan O'Donojú, disembarked on 30 July. He had been instructed to introduce liberal reforms in New Spain but at the same time to ensure that the colony remained within the Spanish empire.