The Portuguese conquest of the Banda Oriental was the armed-conflict that took place between 1816 and 1820 in the Banda Oriental, for control of what today comprises the whole of the Republic of Uruguay, the northern part of the Argentine Mesopotamia and southern Brazil. The four-year armed-conflict resulted in the annexation of the Banda Oriental into the United Kingdom of Portugal, Brazil and the Algarves as the Brazilian province of Cisplatina.

The belligerents were, on one side, the "artiguistas" led by José Gervasio Artigas and some leaders of other provinces that made up the Federal League, like Andrés Guazurary, and on the other, the troops of the United Kingdom of Portugal, Brazil and the Algarves, directed by Carlos Frederico Lecor.

On the naval front, the conflict far exceeded the Río de la Plata and the Argentine coast to spread globally, as the Insurgent privateers, most notably under the flag of Buenos Aires and flag of Artigas, harassed Portuguese ships in Europe, Africa and the Caribbean.

Causes 

The causes that led King John VI of the United Kingdom of Portugal, Brazil and the Algarves, whose court had been installed in Rio de Janeiro since 1808, to embark on the invasion of the Banda Oriental can be divided into general and circumstantial.

Among the first is located in the main place, the former Portuguese aspiration to bring the frontiers of Brazil to the coast of Río de la Plata (), arguing that it matched the Tordesillas line by which Spain and Portugal had divided the world in 1494. For that reason, the region of the Rio de la Plata was a border area between Spain and Portugal, and as such, a highly conflictive area and theater of bloody battles over the centuries, even after the American colonies became independent of the European powers.

The Río de la Plata was strategic because it is the starting point of a large river basin, the fifth in the world, that goes to the heart of South America, from near mining areas in Potosí (current Bolivia), through Paraguay, Mato Grosso and reaching São Paulo. Additionally, the Banda Oriental, in the eighteenth and early nineteenth centuries, was an area of major agricultural wealth, which was organized on the old dairy and beef production, a staple of African slaves who constituted the Brazilian economic base

Following that line of historical conflict, Buenos Aires was founded in 1536 to prevent the Portuguese from extending beyond the Río de la Plata. During the period between 1580 and 1640, in which the Kingdom of Portugal was part of the Iberian Union together with the Kingdom of Spain under the "Catholic Monarchy", Spain relaxed precautions on the ill-defined borders between the two kingdoms, a circumstance that Portugal took to expand the territory of Brazil, to the west and south.

In 1680 the Kingdom of Portugal founded the Colonia del Sacramento (), the first settlement in what is now Uruguay, right in front of Buenos Aires, on the other bank of the Rio de la Plata. Since then several clashes occur and precarious agreements between the Portuguese and Spanish in the Banda Oriental and the Misiones.

Portugal also took the troubled political circumstances produced from Napoleon's invasion of Spain in 1808, introducing the Princess Carlota Joaquina, wife of John VI and sister of King Ferdinand VII, captive of Napoleon, as the best alternative to protect the interests of Spanish crown. However, the common struggle against Napoleon Bonaparte, who invaded Spain in order to attack Portugal, as their government they disregarded the continental blockade imposed by him to Britain, led Portugal to avoid a disagreement with Spain and the occupation projects were delayed.

The occupation of the Misiones Orientales () in 1801 by Portuguese troops, commanded by bandeirante José Francisco Borges do Canto and attempts to generate a protectorate during the crisis of 1808, were closer antecedents. The crisis began when the governor of Montevideo Francisco Javier de Elio came into conflict with the Viceroy of the Río de la Plata, Santiago de Liniers, who came to the political break with the constitution of the Junta of Montevideo on 21 September that year. The Portuguese monarchy took advantage of the situation by sending the military and diplomatic Joaquín Javier Curado to offer, in terms restraining orders, acceptance of the protectorate in the Banda Oriental on the argument the preserve it from a Viceroy considered "Afrancesado". Elio rejected the offer at first, but the course of political events from the 1810 May Revolution in Buenos Aires allowed the Portuguese, on two occasions, to attempt armed seizure of the territory. Those times were 1811 and 1816.

The Portuguese invasion of 1811, was the result of a request by then viceroy of Río de la Plata, Francisco Javier de Elio, in support of the Spanish authorities against artiguist revolutionaries. This invasion took place in the context, as already mentioned, of the May Revolution, where the influence of the same Elío established the capital of the Viceroyalty of the Río de la Plata in Montevideo, thereby becoming viceroy. The revolution had infiltrated in the Banda Oriental after the Cry of Asencio. José Rondeau and José Artigas commanded the troops that, after the Battle of Las Piedras, besieged Montevideo on May 21, 1811. Elio, despite being besieged and in considerable difficulty, managed to block with a royalist naval fleet the port of Buenos Aires and called for help from the Portuguese. A month later, in July, was dispatched from Rio de Janeiro to the south an army of 4,000 men under General Diego de Souza. Defeated in Paraguay and Upper Peru, and stopped by the Elio naval blockade, the government of Buenos Aires sought an agreement with Montevideo in exchange for removing the naval blockade and the withdrawal of the Portuguese. The artiguistas rejected the deal, which left them helpless against the enemy, and followed Artigas in the episode known as the Oriental Exodus. The Portuguese troops had not left the eastern territory until August 1812 when, with the support of the British government, Buenos Aires ensured compliance of the 1811 Armistice Agreement 1811 through the Rademaker-Herrera deal of 1812.

The context of 1816, with the state of war between the Orientals and Buenos Aires (which virtually ensured the neutrality, at least, the neutrality of Buenos Aires to the occupation of the Oriental territory) and the European context, marked by the absolutist restoration that denied the colonies their right to independence from the monarchies (which guaranteed Portugal against any hostile reaction from Spain ), proved ideal for the realization of the old goal. Those were the main circumstances.

The ailing Portuguese royal family that had emigrated to Rio de Janeiro in 1808 fleeing from Napoleon's invasion had nothing to do with the proud Court that concerned Britain in 1816 for its expansionist aspirations. Much water had flowed under the bridges and other winds blowing in Europe and America. The endless possibilities of large and rich country of Brazil, economic development produced by the opening of Brazilian ports to international trade-decision of 1808 - and remoteness from European conflicts resulted in a bold idea of the Portuguese political leadership. Convert Brazil to the center of decision and permanent seat of the kingdom and its authorities. The presence of the Portuguese government in America has substantially changed the geopolitical vision of its leaders. This idea was seriously considered by the king, particularly after the death of his mother, Queen Maria, which occurred in March 1816, who was mentally inhibited from long ago. The Prince Regent finally ascended the throne under the name of John VI. The flamboyant monarch gave his decidedly biased American policy. Brazil seemed to assure the Braganza a first order global destination, which the little Portugal would no longer offer.

A decree almost immediately transformed the Kingdom of Portugal in the United Kingdom of Portugal, Brazil and the Algarves, Brazil was no longer a colony and passed to form the Kingdom of Brazil, essential part of the state. From that moment the expansionist policy was accentuated and the idea of an Empire of Brazil was encouraged and supported. Such a policy did not coincide with the plans of Britain, which was opposed by the British minister Lord Strangford, who previously had a strong influence on the Brazilian government. The dispute culminated when the king himself called to London, in April 1815, to replace the diplomat, which was accepted almost immediately. These circumstances did not alter the old dependency of Portugal, and then of Imperial Brazil, regarding the economic policy of the British Empire. But the relative emancipation of John VI regarding the British power had hitherto closely monitored allowed Portuguese politics execute his old plan to invade and annex the Banda Oriental. Particular interest in the project, had the planters of Rio Grande do Sul, which on the one hand, aspired to control the overseas port of Montevideo as a way to channel their business (the strong regional and even separatist tendencies in the region had the highest interest in having its own exit that would link to international trade), and on the other hand, were concerned about the implementation of rural artiguista regulation, adopted in September 1815, which established the right of confiscation of the lands of the enemies of the revolution with its disrespect for property rights and the phenomenon of rural populace dividing the land, under the banner "The most unhappy are the most privileged". In addition, under the chaos prevailing in the United Provinces, which declared independence after the Congress of Tucumán, and the "radicalism" of Art