The Platine War was fought between the Argentine Confederation and an alliance of the Empire of Brazil, Uruguay, the Argentine provinces of Entre Rios and Corrientes, and Paraguay from 1851 to 1852. The conflict, which occurred in the context of the Argentine Civil Wars and the Uruguayan Civil War, resulted in the overthrow of the Argentine dictator Juan Manuel de Rosas and the start of Brazilian hegemony over the Rio de la Plata.
After seizing power in Buenos Aires in 1829, the Argentine Federalist caudillo Juan Manuel de Rosas strove to recreate the former Viceroyalty of the Rio de la Plata, which would necessitate conquering Bolivia, Uruguay, and Paraguay and incorporating a portion of Brazil's Rio Grande do Sul province. To do so, Rosas backed factions in neighboring countries that were sympathetic to union with Argentina and even financed rebellions and wars. Rosas refused to recognize Paraguayan independence, blockading international traffic to and from Paraguay on the Parana River. Paraguay responded by allying itself with the Empire of Brazil. Carlos Antonio Lopez failed to trigger a rebellion in Corrientes Province in 1845, but, from 1847 to 1850, Paraguay occupied Misiones Province and held their positions despite suffering heavy losses from Bonaerese counterattacks. From 1845 to 1850, the United Kingdom and France punished Argentina's obstruction of international trade by blockading the Rio de la Plata, but Argentine resistance forced the great powers to make peace in 1849.
In 1849, the Brazilian pro-war politician Paulino Jose Soares de Sousa was appointed Foreign Minister, and he made clear his intent to deal with Argentina without European assistance. Brazil sent a contingent south to secure the Platine region, while Brazil allied itself with Bolivia (which agreed to strengthen its border against a potential Argentine invasion, while declining to contribute troops to the conflict) and Paraguay (which also refused to contribute troops, fearing that the anti-Rosas governor Justo Jose de Urquiza was planning to annex Paraguay). On 18 March 1851, Brazil openly declared its support for the Colorado government of Uruguay against the pro-Argentine Blancos, leading to Argentina mobilizing for war.
On 1 May 1851, Urquiza announced that Entre Rios Province intended to exercise its own sovereignty, and Corrientes soon followed suit. Brazil financially supported both uprisings, and the British admiral John P. Grenfell led an armada of 59 Brazilian Navy ships to the Platine region to help with Rosas' overthrow. On 29 May 1851, Uruguay, Brazil, Entre Rios, and Corrientes launched a joint offensive against Rosas. On 2 August 1851, Brazilian troops began to land in Uruguay, and Rosas responded with a declaration of war on 18 August 1851.
On 4 September 1851, 16,200 Brazilian troops crossed the border from Rio Grande do Sul into Uruguay. As the Brazilians moved in, Urquiza and his Colorado allies defeated the Blanco leader Manuel Oribe near Montevideo. The Brazilian Navy prevented Oribe's army fromr etreating into Argentina, and Oribe's defeated men were incorporated into the victorious armies. On 21 November 1851, the anti-Rosas alliance declared a war goal of freeing Argentina from Rosas. On 17 December 1851, the Brazilian Navy forced a passage of the Parana River. In February 1852, the Allies defeated Angel Pacheco at Alvarez Field and Rosas himself at the Battle of Caseros on 3 February. 20,000 Argentines, 2,000 Uruguayans, and 4,000 Brazilian elite troops defeated Rosas' 15,000 cavalrymen, 10,000 infantrymen, and 1,000 artillerymen, and Rosas was forced into exile in Britain. The Allied army proceeded to march through Buenos Aires, with the Brazilians holding their triumphal procession on 20 February, the 25th anniversary of their defeat at the Battle of Ituzaingo. Brazil's intervention secured Paraguayan and Uruguayan independence, while Argentina was unable to invade Rio Grande do Sul. In September 1852, Buenos Aires seceded from Argentina, refusing to accept a new constitution. The civil war would only be brought to an end at the Battle of Pavon in 1861, after which Bartolome Mitre became President of a united Argentine Republic. Continued civil warfare in Uruguay would ultimately spark the Uruguayan War and the Paraguayan War in 1864.