The May Revolution ousted the viceroy Baltasar Hidalgo de Cisneros and replaced him with a junta, the Primera Junta. The change was not limited to the head of government, and started a process of transition from the absolute monarchy to the Republic, in line with the current ideas of the Age of Enlightenment, although other variants like a constitutional monarchy or a Regency were briefly considered. The viceroyalty was also renamed, removing the "Viceroyalty" word from it, and it nominally became the United Provinces of the Ri­o de la Plata. However, the status of the different territories that used to belong to the viceroyalty changed many times during the development of the war, as some regions would remain loyal to former organization, others were captured or recaptured, and later they would divide into several countries.
The first military campaigns against the royalists were waged by Manuel Belgrano and Juan Jose Castelli. The Junta, after expanding itself into the Junta Grande, was replaced by the First Triumvirate. A Second Triumvirate would replace it years later, calling for the Assembly of year XIII that was meant to declare independence and write a constitution. However, it did not do either thing, and replaced the triumvirates with an unipersonal head of state office, the Supreme Director.
By this time Jose de San Marti­n arrived to Buenos Aires with other generals of the Peninsular War. They gave new strength to the Revolutionary war, which was compromised by the defeats of Belgrano and Castelli and the royalist resistance at the Banda Oriental. Alvear took Montevideo, and San Marti­n started a military campaign that would span across an important part of the Spanish territories in America. He created the Army of the Andes in Mendoza and, with help of Bernardo O'Higgins and other Chileans he made the Crossing of the Andes and liberated Chile. With the Chilean navy at his disposal, he moved to Peru, liberating that country as well. San Marti­n met Simon Bolivar at Guayaquil, and retired from action.
A new assembly, the Congress of Tucuman, was called while San Marti­n was preparing the crossing of the Andes. It finally declared independence from Spain or any other foreign power. Bolivia declared itself independent in 1825, and Uruguay was created in 1828 as a result of the Argentina-Brazil War.
The United Kingdom officially recognized Argentine independence in 1825, with the signing of a Treaty of Friendship, Commerce, and Navigation on February 2; the British charge d'affaires in Buenos Aires, Woodbine Parish, signed on behalf of his country. Spanish recognition of Argentine independence would take more decades to take place.