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.

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.

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.

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".

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, through the Rademaker-Herrera deal of 1812.

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.

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.

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 Artigas, he regarded the Banda Oriental as a dangerous center spread of "anarchy" to impulses of the "Montoneros" federalist and republican.

Also, Spanish and American emigrants who sought refuge in Brazil persuaded the Portuguese and Brazilian King John VI, to initiate a military campaign on the Banda Oriental.

The locals unit exiled by the Fontezuelas Mutiny led by Carlos de Alvear, expected a defeat of Artigas, leader of federalism, and supplied the Portuguese-Brazilian Court with any information, to support their plans.

The main drawback of the campaign of conquest lay in the possibility that the United Provinces of the Río de la Plata react in defense of a territory which formed part of the country since its inception.

According to Uruguayan historians Washington Reyes Abadie, Oscar H. Bruschera and Tabaré Melogno, and the Argentine Raúl Scalabrini Ortiz, the information about Buenos Aires neutrality was first given by Manuel José García, sent to negotiate with Britain and the court in Rio de Janeiro that they do not support the Spanish Empire, at a time when it was trying to recover the independent colonies.

According to the Uruguayan writer Lincoln Maiztegui Casas, "Garcia with his ideal and interventional unit used all his influence to persuade the Portuguese King that the government of Buenos Aires would not take military action to keep the Oriental territory."

Ultimately, Pueyrredón as supreme director, collaborated with the invasion, not only because he did not declare war on the United Kingdom of Portugal, Brazil and the Algarves to the occupation of part of the country he governed, but because he steadily attacked the provinces of the Federal League, which were inhibited to cooperate with the defense of the territory organized by Artigas.

Pueyrredón, expressing a political line that held differences with predominantly Unity Party in Buenos Aires since 1812, and was taken with considerable concern the Portuguese invasion of the Banda Oriental, with no objective reasons to ensure that such attitude was apparent.

First, Nicolas de Vedia sent to interview Lecor, leading a trade to ensure that the invasion did not continue to Entre Rios, but that he had instructions to "keep on Buenos Aires strict neutrality".

The "Instructions of His Majesty King John VI" to General Lecor, signed by the Marquis de Aguilar, the June 4, 1816 are particularly illustrative for understanding the motives and purposes of the Portuguese action in the Platine territories.

Finally, it was recommended to keep strict neutrality with the government of Buenos Aires, and in the case of being asked he would deny any intention of moving to the other side of the Rio de la Plata.

After the Division of General Bernardo Silveira invaded Cerro Largo, it headed towards Paysandú with the mission to protect the right flank of Lecor with their 2,000 men of the Royal Volunteer Cavalry and 800 riograndense militias.