The Platine War (, ; 18 August 1851 – 3 February 1852) was fought between the Argentine Confederation and an alliance consisting of the Empire of Brazil, Uruguay, and the Argentine provinces of Entre Ríos and Corrientes, with the participation of the Republic of Paraguay as Brazil's co-belligerent and ally. The war was part of a decades-long dispute between Argentina and Brazil for influence over Uruguay and Paraguay, and hegemony over the Platine region (areas bordering the Río de la Plata). The conflict took place in Uruguay and northeastern Argentina, and on the Río de la Plata. Uruguay's internal troubles, including the longrunning Uruguayan Civil War (La Guerra Grande – "The Great War"), were heavily influential factors leading to the Platine War.
In 1850, the Platine region was politically unstable. Although the Governor of Buenos Aires, Juan Manuel de Rosas, had gained dictatorial control over other Argentine provinces, his rule was plagued by a series of regional rebellions. Meanwhile, Uruguay struggled with its own civil war, which started after gaining independence from the Brazilian Empire in 1828 in the Cisplatine War. Rosas backed the Uruguayan Blanco party in this conflict, and further desired to extend Argentine borders to areas formerly occupied by the Spanish Viceroyalty of the Río de la Plata. This meant asserting control over Uruguay, Paraguay, and Bolivia, which threatened Brazilian interests and sovereignty since the old Spanish Viceroyalty had also included territories which had long been incorporated into the Brazilian province of Rio Grande do Sul.
Brazil actively pursued ways to eliminate the threat from Rosas. In 1851, it allied with the Argentine breakaway provinces of Corrientes and Entre Ríos (led by Justo José de Urquiza), and the anti-Rosas Colorado party in Uruguay. Brazil next secured the south-western flank by signing defensive alliances with Paraguay and Bolivia. Faced with an offensive alliance against his regime, Rosas declared war on Brazil.
Allied forces first advanced into Uruguayan territory, defeating Rosas' Blanco party supporters led by Manuel Oribe. Afterwards, the Allied army was divided, with the main arm advancing by land to engage Rosas' main defenses and the other launching a seaborne assault directed at Buenos Aires.
The Platine War ended in 1852 with the Allied victory at the Battle of Caseros, for some time establishing Brazilian hegemony over much of South America. The war ushered in a period of economic and political stability in the Empire of Brazil. With Rosas gone, Argentina began a political process which would result in a more unified state. However, the end of the Platine war did not completely resolve issues within the Platine region. Turmoil continued in subsequent years, with internal disputes among political factions in Uruguay, a long civil war in Argentina, and an emergent Paraguay asserting its claims. Two more major international wars followed during the next two decades, sparked by territorial ambitions and conflicts over influence.
Background
Rosas rule in Argentina
Don Juan Manuel de Rosas became governor of Buenos Aires after the brief period of anarchy following the end of the Cisplatine War in 1828. In theory, Rosas only held as much power as governors of the other Argentine provinces, but in reality, he ruled over the entire Argentine Confederation, as the country was then known. Although he was one of the Federalists, a faction which demanded greater provincial autonomy, in practice Rosas exercised control over the other provinces and become the virtual dictator of Argentina. During his 20-year government, the country witnessed the resurgence of armed conflicts between the Unitarians (his rival political faction) and the Federalists.
Rosas desired to recreate the former Viceroyalty of the Río de la Plata. He aimed to build a powerful, republican state with Argentina placed at the center. The defunct Viceroyalty had shattered into several separate nations following the Argentine War of Independence at the beginning of the 19th century. To achieve reunification, the Argentine government needed to annex the three neighboring countries – Bolivia, Uruguay and Paraguay, as well as to incorporate a portion of the southern region of Brazil. Rosas first had to gather allies across the region who shared his vision. In some instances, this meant that he had to become involved in the internal politics of neighboring countries, backing those sympathetic to union with Argentina, and occasionally even financing rebellions and wars.