In 1816, the United Kingdom of Portugal, Brazil and the Algarves carried out an invasion of the Banda Oriental and, after defeating the local resistance led by José Gervasio Artigas, annexed it under the name of Cisplatina.

After a series of initial skirmishes, they defeated the Brazilians at the battles of Rincón and Sarandí, prompting the Argentine Congress to proclaim Cisplatina reintegrated into the United Provinces on 25 October.

The economic burden and internal political disputes caused by the war in both states, especially the Brazilian naval blockade and the impasse on land, led both countries to start peace negotiations.

The conflict continued until 27 August 1828, when Argentine and Brazilian representatives, under British mediation, signed the Preliminary Peace Convention of 1828, by which Cisplatina would become an independent state and hostilities would cease.

[7] By expanding into the River Plate basin, the Portuguese wanted to secure access to the continent's interior and also divert the smuggling of silver from Upper Peru away from Buenos Aires.

[10] With the outbreak of the 1810 May Revolution in Buenos Aires and the Argentine struggle for independence that ensued, Montevideo, under the command of viceroy Francisco Javier de Elío, remained loyal to Spain.

[11] The Primera Junta of Buenos Aires then set out to subdue Montevideo and Elío, who, finding himself without support from Europe, requested assistance from Carlota Joaquina, the Spanish wife of Portuguese prince regent John of Braganza.

[24] On 31 July 1821, the Montevideo cabildo, with representatives from all of the Banda Oriental, approved its incorporation into the United Kingdom of Portugal, Brazil and the Algarves as a province under the name of Cisplatina.

[26][e] Brazilian patriots, loyal to prince Pedro and under the command of Carlos Frederico Lecor, then besieged and blockaded the Portuguese troops garrisoned in Montevideo, finally defeating them in 1824.

[29] Following the United Province's recognition of Brazil's independence on 25 June 1823, the country immediately began diplomatic talks with the Empire regarding Cisplatina, which the Argentine government considered theirs and wanted to gain possession of.

[32] The answer was given on 6 February 1824, in which the Imperial government promptly rejected any negotiations regarding the cession of Cisplatina, concluding its note by declaring:[33] Therefore, on these important considerations, the Government of His Imperial Majesty can not enter with Buenos Aires on a negotiation which has for its fundamental basis the cession of the Cisplatine State, the inhabitants of which it can not abandon.Emperor Pedro I of Brazil, who had been informed of the situation in Cisplatina by Lecor, believed the natives of the province wanted to remain as part of the Empire; however, Lecor had surrounded himself with people who were sympathetic to Brazil, which prevented him from knowing the true intentions of the province's inhabitants.

Despite having distinguished himself in the service of the Empire and having received several promises of career advancement, Rivera decided to revolt, but continued to disguise his intentions from the Brazilian Court.

[39][i] On 19 April 1825, a group of 33 men known as the Treinta y Tres, or the Thirty-Three,[j] led by Juan Antonio Lavalleja, left Buenos Aires, crossed the Uruguay River and landed on Agraciada beach with arms and ammunition, starting the rebel movement against the Empire of Brazil; the action had Rivera's knowledge and collusion.

[46] The Argentine government replied that it had no part in the rebellion and that "the supplies furnished from Buenos Aires had been bought either with the money, or on the credit, of private individuals in the stores of the city, which were open to all alike, whether friends or enemies".

On 17 August, a new battle took place near Colonia del Sacramento, when colonel João Ramos, at the head of 300 Brazilians, fought against 400 Uruguayans, who, after a fierce combat, retreated with considerable losses.

[54] According to Carneiro, the victory made Ribeiro too confident and anxious: after defeating Rivera, he marched to Montevideo in order to convince Lecor, who passively watched everything, to take the initiative.

[70] Toscano estimated the Brazilian fleet at no less than 65 large warships for a total of 690 guns in 1825, also counting 31 small armed packet boats and transport vessels, noting that it had doubled in size in the three years after the country's independence war by the purchase or incorporation of ships that had been captured by lord Thomas Cochrane from Portugal during the conflict.

[71] Its quality varied, however; the frigates were no older than eight years, but the corvettes and brigs included ships from the Napoleonic Wars, such as Itaparica, Liberal and Cacique, to new ones built in North America, such as Maria da Gloria and Maceió.

The former, wanting to harm the emperor's ministers, described the navy as a "naval police", saying that, instead of the large and heavy frigates bought by the government, it would have been better to acquire low draft schooners, better suited for navigation in the shallow waters of the River Plate.

[72] For John Armitage, the desire of "ostentation" and to show off an "exalted idea" of its naval power to foreign nations had led Brazil to purchase and build heavy vessels, unsuited for war in the River Plate.

[78] In Jorge Toscano's words, when compared to Brazil's maritime power, the Argentine Navy was "insignificant and consisted of little more than a handful of small ships left aside since the independence campaign", also remarking that the sight of admiral Lobo's fleet anchored in front of Buenos Aires was a constant reminder of the country's vulnerability.

But by 1828, the superior numbers of Brazil's blockading squadrons had effectively destroyed Brown's naval force at the Monte Santiago and was successfully strangling the trade of Buenos Aires and the government revenue it generated.

Moreover, ever since the United Provinces' independence from Spain, Buenos Aires had become a centre of trade, which, being carried out mostly by British, American, and French ships, totalled a yearly sum of 2 million pounds in 1825.

Woodbine Parish, the British consul in the city, protested that it would be impossible for neutral vessels to leave in time, and so admiral Lobo agreed to postpone the beginning of the blockade to 31 January 1826.

[90] Likewise, the American consul in Buenos Aires, colonel John Forbes, protested and accused Brazil of conducting a "paper" blockade for breaking the requirements of presence and continuity, since Lobo's fleet had made frequent returns to Montevideo for long periods.

[92] In May 1826, Rodrigo Lobo was replaced by admiral Pinto Guedes, who reinvigorated the blockade, causing an enormous increase in the number of vessels seized, which motivated protests from Britain, France and the United States.

With foreign patience waning, the Brazilian government backed down: on 26 November 1826, Guedes was ordered only to seize neutral vessels that were attempting to run the blockade after individually warning each one of them.

[94] Disappointed with his government's decision, Guedes declared: "I feel not the least repugnance to act fairly to the subjects of His Britannic Majesty, but no-one is ignorant of the mischief which citizens of the United States have done by taking advantage of the obstructions placed in the way of this squadron".

[97] Bonds issued for public and private investments lost their value and inflation quickly went up, as the government used its metal reserves to finance the war, leaving fiat money unbacked.

Under British mediation, the United Provinces and the Empire of Brazil signed the 1828 Treaty of Montevideo, which acknowledged the independence of Cisplatina under the name Eastern Republic of Uruguay.