The Anglo-French blockade of the Río de la Plata, also known as Paraná War, was a five-year naval blockade imposed by France and the United Kingdom on the Argentine Confederation during the Uruguayan Civil War. It was imposed by the Royal Navy and French Navy in 1845 against the Río de la Plata Basin to support the Colorado Party in Uruguay's civil war, resulting in the closure of Buenos Aires to maritime commerce. The Argentine government, led by Juan Manuel de Rosas, refused to drop their support for the Uruguayan White Party, which supported Argentina's resistance to the blockade. Eventually, both Britain and France ended the blockade, signing the Arana–Southern Treaty in 1849 and 1850 respectively, which acknowledged Argentine sovereignty over its rivers.
Background
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, which 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 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 itself from Argentina.
International context
Britain did not have great interests at stake in Buenos Aires. The purpose of the war was to foster friendly diplomatic relations 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.
Two influential French politicians of the time were the foreign minister François Guizot and the nationalist Adolphe Thiers. Thiers proposed that France should continue the hostilities against Rosas. He explained his opinion with three main reasons and an accessory one: humanity, patriotism, international law and expansion of commerce. He viewed Rosas as a ruthless dictator, to justify the humanitarian reason. Although Montevideo was not a French colony, he referred to it as such due to its high population of French Basques. He considered that Rosas would be in contravention of the 4th article of the Mackau-Arana treaty (in which the Confederation recognized the independence of Uruguay) by attempting to impose an Argentine president on it. The expansion of commerce was not mentioned as a main reason, as Rosist protectionist policy limited but did not forbid French imports. Guizot rejected these ideas. He did not consider that Rosas contravened the Mackau-Arana treaty because the Confederation was not actually at war with Uruguay, but was merely supporting one Uruguayan faction against the other. He did not think that the French economy could be further benefited by taking action in La Plata, nor that it was workable to keep a colony in the zone. In respect to humanitarian reasons, he dismissed the purported Rosist crimes because they were based on reports by the Unitarian Florencio Varela, who had no reputation as an impartial reporter. However, Guitoz ultimately agreed to the proposal, in order to foster an alliance with Britain and reduce the popular acclaim of Thiers.
Brazilian viscount Miguel Calmon du Pin e Almeida met the British Lord Aberdeen to request that Brazil join Britain and France in the action against Rosas. But Britain did not have good relations with Brazil at that time, so Aberdeen rejected the Brazilian aid. Aberdeen complained that, if Brazil thought that civilization was tied with free trade, Brazil should accept extending their free trade agreement with Britain before imposing it on Rosas, and that if Brazil was concerned with humanitarian reasons they should start by abolishing slavery in their own country. Then he met Guizot, who was not receptive either. Guizot felt forced by circumstances to join a war where he did not expect to earn anything, and was reluctant to involve French troops in it. Britain would not resort to ground troops either, as the British invasions of the Río de la Plata had proved them ineffective. The viscount informed the Brazilian court of the Anglo-French resolution. The public purposes of the war were to protect the Uruguayan independence against Oribe, defend the recently proclaimed independence of Paraguay, and end the civil wars at La Plata. He also revealed the secret purposes: To turn Montevideo into a "commercial factory", to force the free navigation of the rivers, to turn the Argentine Mesopotamia into a new country, to set the borders of Uruguay, Paraguay and the Mesopotamia (without Brazilian intervention), and to help the antirosistas to depose the governor of Buenos Aires and to establish one loyal to the European powers. For the viscount, it was an all-losing situation for Brazil: if the intervention was defeated, they would be helpless against the counter-attack of Rosas, Oribe and perhaps the Ragamuffins; and if the intervention prevailed, Brazil would have neighbors even more powerful than the Confederation. Thus, he strongly advised against joining it, and restoring good relations with Rosas if possible.
William Brent, representative of the United States in Buenos Aires and supporter of the Monroe Doctrine, supported Rosas in the dispute with the European powers, and proposed to act as mediator. After the victory at the battle of India Muerta, Oribe was about to enter Montevideo. Brent proposed that he did so giving an amnesty to the supporters of Rivera, calling to elections in the Legislature, and then the Argentine forces supporting him should leave the country at that point. Oribe accepted, but the British arrived at Montevideo and refused to allow Oribe to enter the city. Arana, Deffaudis and Ouseley rejected Brent's mediation, and gave an ultimatum: if the army of Oribe and the Argentine navy of William Brown did not leave Uruguay in ten days, they would proceed to blockade Buenos Aires.
Beginning of armed actions
The same night the ultimatum was issued, still during the ten days period, the British ship Cadmus and the French one D'Assas anchored next to the Argentine ships San Martín and 25 de mayo. William Brown requested permission to return to Buenos Aires, which was allowed but with the condition of delivering the British and French sailors. Brown requested permission to do so in Buenos Aires, as most of his crew was Irish, but received no answer. When he finally sailed back to Buenos Aires, he was attacked. The Anglo-French forcibly boarded and seized the complete Argentine navy, the ships San Martín, 25 de mayo, General Echagüe, Maipú and 9 de julio. The French raised their flag in the first two, and the British did so in the last three. The Argentine officers were returned to Buenos Aires, and William Brown and all the British sailors were forbidden to sail under the Argentine flag during the remainder of the conflict.
The following day, the Anglo-French forces disembarked in Montevideo, reinforcing the defenses of the city. Since the defeat at India Muerta, the Montevidean defenders were less than 3500. Rivera expressed gratitude for these actions, saying that they had secured Uruguayan independence. Many politicians of Buenos Aires criticized them during a meeting in the Junta of Representatives in Buenos Aires.
Rosas learned in 1838 that the Isla Martín García was difficult to defend, so he removed the forces from it. However, to prevent foreign claims of sovereignty, he left a force of twelve aged crippled soldiers, who would keep the flag of Argentina raised until the end. The only purpose of this army was to assert that the island was not left abandoned, and that the Anglo-French forces would only take it by an invasion. General Lucio Mansilla received the forces and infrastructure of Martín García, to select a point in the Paraná River and fortify it. The west side of the river was preferred, to have easier communications with Buenos Aires. However, the support to the Uruguayan Oribe took priority, and no Argentine forces supporting the siege would be retired from it, not even temporarily. Mansilla prepared the defenses at Obligado, Buenos Aires, near San Pedro.
Declaration of blockade
The blockade was formally declared on 18 September 1845. They cited many reasons. They said that Rosas did not stop the war despite their good intentions, or that the capture of the Argentine navy, invasion of Martín García and reinforcement of Montevideo were described in violent language at Buenos Aires newspapers, which was also found at the meeting of the Junta or in the messages that called the Unitarians savages. A decree of 27 August had forbidden all Argentines to communicate with the Anglo-French navy. They also said that the foreigners in Buenos Aires were abused and drafted into the army, that Oribe made a butchery after the victory at India Muerta, and that the police was headed by the Mazorca, which would make several abuses. The tone was closer to that of a declaration of war, and may have been written by Florencio Varela.
To counter those claims, Rosas arranged a meeting with diplomats from the United States, Portugal, Sardinia, Bolivia and France. The British diplomat refused to assist, but the French Mareuil did so. All of them declared unanimously that they had no complaints about the treatment to foreigners, that they had no knowledge of foreigners forced by terror into the military or to sign petitions, that they had no knowledge of abuses from the Mazorca, and that the information of alleged butcheries in India Muerta was inexact. Rosas included as well a petition signed by 15 000 British and French living in Buenos Aires, protesting against the blockade. Rosas was confident in that this formal declaration, signed by foreign diplomats, would counter the Montevidean propaganda and turn the international opinion to his side.