The Anglo-French blockade of the Río de la Plata or also known as War Of Parana was a five-year-long naval blockade imposed by France and Britain on the Argentine Confederation ruled by Juan Manuel de Rosas. It was imposed in 1845 to support the Colorado Party in the Uruguayan Civil War and closed Buenos Aires to naval commerce. The Anglo-French navy trespassed into Argentina's internal waters to sell their products, as Rosas maintained a protectionist policy to improve the weak Argentine economy. Eventually both Britain and France gave in, signing treaties in 1849 (Britain) and 1850 (France) acknowledging the Argentine sovereignty over its rivers.
Antecedents
Local context
Buenos Aires faced the French blockade of the Río de la Plata between 1838 and 1840. The Peru–Bolivian Confederation, allied with France, declared the War of the Confederation on Argentina. Rosas resisted the blockade longer than France estimated he would do, and his strategy of generating disputes between France and England over the blockade eventually bore fruit. France lifted the blockade in 1840, exchanging mutual most favoured nation status between her and the Argentine Confederation.
Unable to deploy French troops during the blockade, France promoted civil wars against Rosas to support the naval actions. For this purpose, France aided Fructuoso Rivera against the Uruguayan president Manuel Oribe, who was forced to resign. Oribe escaped to Buenos Aires, and Rosas received him as the legitimate president of Uruguay, denying such recognition to Rivera. This started the Uruguayan Civil War, where the Blancos sought to restore Oribe in power and the Colorados to keep Rivera. As Rivera was hesitant to attack Rosas as the French expected, the Argentine expatriate Juan Lavalle was convinced to do so, but his army bolstered by French troops was nevertheless weakened by desertions and hostility from the local population on their march to Buenos Aires, and French monetary support was curtailed, as France conducted negotiated peace with Rosas by that time. Lavalle's army retreated to the north in disorder, without attacking Buenos Aires as intended.
Rivera's ambition was to expand the borders of Uruguay, annexing Paraguay, the Argentine Mesopotamia and the Riograndense Republic (part of Rio Grande do Sul, that had declared independence from Brazil and was fighting the Ragamuffin War), into a projected Federation of Uruguay. The Argentine José María Paz, allied with Rivera against Rosas, was against this project. Rivera took control of Paz's forces, but without his superior military training, he was completely defeated by Oribe at the battle of Arroyo Grande. Rivera's project never got off the ground, and he was forced to stand in Montevideo against Oribe's siege.
Brazil proposed a military alliance to Rosas: Rosas would take Uruguay with Brazilian support, and Brazil would take the Riograndese Republic with Argentine support. Tomás Guido, Argentine representative in Brazil, supported the proposal, but Rosas rejected it. Rosas thought that such treaty would violate the Uruguayan sovereignty, and that it would be null if Oribe was not part of it. Honório Carneiro Leão, representative of Brazil, did not accept Rosas's alternative proposal, and Brazil distanced from Argentina.
International context
Britain did not have great interests at stake in Buenos Aires. The purpose of the war was to foster the Entente cordiale with France, so as to make possible later joint military operations elsewhere, such as the Opium War or the protection of the independence of the Republic of Texas. The British interests in South America grew when Texas was finally annexed by the United States. Texas supplied Britain with cotton, and the British calculated that it would be easier to secure cotton sources in South America (such as Paraguay) rather than waging war with the powerful United States. Britain republished many Uruguayan libels against Rosas, such as the Blood tables, so that people supported military action against him. As there was a strong British religious tradition, Rosas was accused of replacing the crosses of churches with his portrait, persecuting religion, and killing priests; and that he may have murdered his wife Encarnación Ezcurra and had incestuous relations with his daughter.