Cisplatine War The Cisplatine War was an armed conflict over an area known as Banda Oriental or the " " Eastern Shore " " ( roughly present - day Uruguay ) in the 1820s between the United Provinces of the Río de la Plata and the Empire of Brazil in the aftermath of the United Provinces ' emancipation from Spain . Background . Led by José Gervasio Artigas , the region theretofore known as the Eastern Bank , in the River Plate basin , revolted against Spanish rule in 1811 , against the backdrop of the 1810 May Revolution in Buenos Aires as well as the regional rebellions that followed in response to Buenos Aires ' pretense of primacy over other regions of the viceroyalty . In the same context , the Portuguese Empire , then hosted in Rio de Janeiro , took measures to solidify its hold on Rio Grande do Sul and to annex the region of the former Eastern Jesuit Missions . From 1814 on , the Eastern Province joined forces with the provinces of Santa Fe and Entre Rios in a loose confederation called the Federal League , which resisted Buenos Aires ' authority . After a series of banditry incidents in territory claimed by the Portuguese Empire , the Rio Grande do Sul , Portugal invaded the Eastern Bank in 1816 . Artigas was finally defeated by the Luso - Brazilian troops in 1820 at the Battle of Tacuarembó . The Portuguese Empire ( formally the United Kingdom of Brazil , Portugal and Algarves since 1815 ) then formally annexed the Eastern Bank , under the name Cisplatine Province , with support from local elites . With the annexation , the Portuguese Empire now enjoyed strategic access to River Plate and control of the estuary ' s main port , Montevideo . After Brazilian independence , in 1822 , the Cisplatine province became part of the Empire of Brazil . It sent delegates to the 1823 Constitutional Convention and , under the 1824 Constitution , enjoyed a considerable degree of autonomy , more so than other provinces of the Empire . Conflict . While initially welcoming Portuguese intervention in the rogue Eastern province , the United Provinces of River Plate eventually urged the local populace to rise up against Brazilian authority , giving them political and material support with a view to reestablishing sovereignty over the region . Rebels led by Fructuoso Rivera and Juan Antonio Lavalleja carried on resistance against Brazilian rule . In 1825 , a Congress of delegates from all over the Eastern Bank met in La Florida and declared independence from Brazil , while reaffirming its allegiance to the United Provinces of River Plate . In response , Brazil declared war on the United Provinces . Brazilian Emperor Pedro I ordered his fleet to block the River Plate and its two main ports , Buenos Aires and Montevideo . The Argentina fleet moved south , first to Ensenada and then to distant Carmen de Patagones on the Atlantic Ocean . The Brazilian fleet attempted to take Carmen de Patagones in 1827 and thus tighten its blockade over Argentina , but Brazilian troops were eventually repelled by local civilians . On land , the Argentine army crossed the River Plate and established its headquarters near the Uruguayan town of Durazno . General Carlos María de Alvear invaded Brazilian territory and a series of skirmishes followed . Emperor Pedro I planned a counteroffensive by late 1826 , and managed to gather a small army mainly composed of Southern Brazil voluntaries and European mercenaries . The recruiting effort was hampered by local rebellions throughout Brazil , which forced the Emperor to relinquish direct command of his Army , return to Rio de Janeiro and bestow command of the troops on Felisberto Caldeira Brant , Marquis of Barbacena . The Brazilian counteroffensive was eventually stopped at the inconclusive Battle of Ituzaingó . While Brazilian troops were prevented from marching on to Buenos Aires , Argentine troops no longer managed to operate in Brazilian territory . Ituzaingó was the only battle of some magnitude in the whole war . A series of smaller clashes ensued , including the Battle of Sarandí , and the naval Battles of Juncal and Monte Santiago . Scarcity of volunteers severely hampered Brazilian response , and by 1828 the war effort had become extremely burdensome and increasingly unpopular in Brazil . That year , Rivera reconquered the territory of the former Eastern Jesuit Missions . Aftermath . The stalemate in the Cisplatine War was caused by the inability of the Argentine and Uruguayan land forces to capture major cities in Uruguay and Brazil , the severe economic consequences imposed by the Brazilian blockade of Buenos Aires , and the lack of manpower for a full - scale Brazilian land offensive against Argentine forces . There was also increasing public pressure in Brazil to end the war . All of this motivated the interest on both sides for a peaceful solution . Given the high cost of the war for both sides and the burdens it imposed on trade between the United Provinces and the United Kingdom , the latter pressed the two belligerent parties to engage in peace negotiations in Rio de Janeiro . Under British and French mediation , the United Provinces of River Plate and the Empire of Brazil signed the 1828 Treaty of Montevideo , which acknowledged the independence of the Cisplatine Province under the name Eastern Republic of Uruguay . The treaty also granted Brazil sovereignty over the eastern section of the former Eastern Jesuit Missions and , most importantly , guaranteed free navigation of the River Plate , a central national security issue for the Brazilians , but also an issue of great interest to the United Kingdom . In Brazil , the loss of the Cisplatine Province added to growing discontent with Emperor Pedro I . Although it was far from the main reason , it was a factor that led to his abdication in 1831 . Legacy . Although the war was not a war of independence , as none of the belligerents fought to establish an independent nation , it has a similar recognition within Uruguay . The Thirty - Three Orientals are acknowledged as national heroes , who freed Uruguay from Brazilian presence . The landing of the Thirty - Three Orientals is also known as the " Liberation crusade " . The war has a similar reception within Argentina , considered as a brave fight against an enemy of superior forces . The Argentine Navy has named many ships after people , events and ships involved in the war . William Brown ( known as " Guillermo Brown " in Argentina ) is considered the father of the Argentine navy , and is treated akin to a epic hero for his actions in the war . He is also known as the " Nelson of the Río de la Plata " . Brazil has had little interest in the war beyond naval warfare buffs . Few Brazilian historians have examined it in detail . The national heroes of Brazil are instead from the Brazilian independence , the conflicts with Rosas or the War of the Triple Alliance . Despite the role of Britain in the war , and the presence of British naval officials on both sides of the conflict , the war is largely unknown in the English - speaking world .