Anglo - French blockade of the Río de la Plata The Anglo - French blockade of the Río de la Plata 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 the internal waters of Argentina , in order 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 acknowledging the Argentine sovereignty over its rivers in 1849 ( Britain ) and 1850 ( France ) . 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 to Argentina and Chile . 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 gave 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 war s 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 Whites 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 was weakened by desertions , hostility of the villages in the way to Buenos Aires , and the lack of French support , as France had 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 limits 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 . 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 meet 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 ended 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 to 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 to Montevideo and refused to allow Oribe to enter to 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 block 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 forced 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 3 500 . Rivera thanked these actions , saying that it secured the 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 September 18 , 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 August 27 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 made 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 complains 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 . Durán de Mareuil , representative of French business in Buenos Aires , was among the signatories . He wrote a document requesting the end of the blockade , which included Rosas ' s demands . Those demands were the inclusion of Oribe in the negotiations , disarm of Montevideo , return of Colonia , Martín García and the stolen navy , departure from the internal rivers , acknowledgement of the sovereignty of Argentina and an indemnization . As expected , it was rejected in Montevideo , so Mareuil moved to Paris to give it directly to the French government . Uruguay river . The Anglo - French navy navigated the internal Uruguay river by mid August , led by Lainé and Inglefield . They announced that they would block any ports supporting Oribe , and remove the people in them with gunshots . In response , the ports were closed to any communication with the Anglo - French navy . Lainé and Inglefield moved to Colonia del Sacramento , with Giuseppe Garibaldi and his legion of Italian volunteers . They had a total of 28 ships , whereas Jaime Montoro , colonel defending the city had only 300 soldiers and eight small cannons . The Italian legion disembarked in the city and pillaged it . José Luis Bustamante blamed Garibaldi for it , while Garibaldi would attribute it to a lack of military discipline among his Legion . He would write in his memoirs that " " the repression of disorder was difficult , considering that Colonia had plenty of resources , and specially of spirituous liquids that increased the desires of the virtuous pillagers " " . Even the local church was sacked , and the drunk Italians spent the night in it . The navy moved then to Martín García , defeating the army stationed there . The flag of Argentina was removed , and replaced with the flag of Uruguay . the Argentine soldiers were removed , and the island was left abandoned . The ship moved then into the Uruguay river . The pillage of Gualeguaychú was even worse than in Colonia , to the point that Bustamante wrote to Rivera worried about Garibaldi , fearing that his actions would discredit the whole operation . Garibaldi described that " " The city of Gualeguaychú drove us to conquest by being a real emporium of wealth , capable to dress our ragged soldiers and supply us with harnesses . We acquired many and very good horses in Gualeguaychú , the clothes needed to dress all the people , the harnesses of cavalry and some money distributed among our poor soldiers and sailors , who suffered so much time of misery and privations " " . Garibaldi ' s plundering was valued in nearly 30 , 000 pound sterlings . Garibaldi was defeated at Paysandú by colonel Antonio Díaz , and then at Concordia , defended by Juan Antonio Lavalleja and an improvised navy . Then , he took control and pillaged Salto . By November , the Anglo - French navy had control of all the Uruguay river from Colonia to Salto . Parana river . Once Montevideo had enough defenses , Ouseley and Defauis prepared a convoy to navigate the Parana river . This way they would reach Corrientes and Paraguay and , once having complete control of Uruguay and both rivers , force the Mesopotamia out of the Confederation . The convoy was composed of three steamboats , capable to navigate independently from the winds and a number of heavily armed sailboat s . Those ships would protect 90 merchant vessels , of diverse nationalities . Arana notified the foreign diplomats that the ships entering the Paraná unauthorized would be reputed as pirates , so they informed their respective ships that they would not have protection from their home countries if they got involved in the operation . However , such messages were delayed , and the 90 ships took part of the operation anyway . The Anglo - French weaponry was the most advanced of the time . They used " Peysar " rifled cannons , and the French brought the new Paixhans gun s . The allies also bombarded the Confederation batteries with Congreve rocket s . This would be the first time such weapons would be used in South America , and they expected their firepower to be devastating . The convoy stopped at the Paraná Guazú channel to study the situation . Initially , the admirals though that they would navigate unopposed , but found that Lucio Mansilla had prepared many fortifications along the river . As a result , the merchant ships would stay behind , while the combat steams open the way . There were fortifications at Ramada , Tonelero , Acevedo , and San Lorenzo . The most important fortification was located in Obligado , near San Pedro . Battle of Vuelta de Obligado . In Obligado the Parana is only 700 m . wide , and the turning made sailboat navigation difficult . Mansilla placed 24 boats in succession , holding three thick chains to close the river . The west coast was the only one fortified , with four batteries . The biggest Argentine cannons were of caliber 20 , whereas the average in the Anglo - French navy was of 80 . The land was defended by the Regiment of Patricians , and the volunteers from the countryside were led by Facundo Quiroga ( son of the famed caudillo of the same name ) . Many artilleries were operated by British sailors from the captured Argentine fleet , who disobeyed the orders of not fighting against their home country . A brig of six cannons was the only Argentine combat vessel . The first steam boats arrived to Obligado on November 18 , stopping beyond the range of the cannons . They waited for the stolen " San Martín " ship , captained by Trehouart , who arrived the following day . The attack was delayed one more day , because the rain did not allow a clear sight of the fortifications . The ships advanced on November 20 . Lucio Mansilla arranged the troops saying : " There they are ! Consider the insult they make to the sovereignty of our nation by navigating , with no more title than force , the waters of a river that flows across the territory of our country . But they won ' t achieve that unpunished ! Let the blue and white banner wave in the Parana , and let ' s all of us die before seeing it come down from where it waves ! " The first ship to advance was the " San Martín " . She was about to break the chains when the wind suddenly ended and she got stuck in the place , too far from the other ships , which could not got near because of the lack of wind . The " San Martín " was hit more than a hundred times , two cannons were destroyed , and two officers and forty sailors died . Finally , the chain of " San Martín " ’ s anchor was broken , and she moved down the river . The " Dolphin " and " Pandour " had to retreat as well . When the " Republicano " ran out of ammunition , the captain blew it to prevent it from being captured . At this point , the steamboats ( unaffected by the lack of wind ) proceeded to the chains . Their powerful guns outranged the Argentine cannons . The " Fulton " got to the chains and broke them , and the wind blew again . The ships moved , and the defenses gradually ran out of ammunition as well . By the end of the day , all batteries were destroyed , and the cannons were destroyed or taken as trophies . 250 Argentine soldiers died , and 400 were injured . The Anglo - French fleet stayed 40 days in Obligado , making repairs . Consequences of Obligado . Word of the actions in Obligado spread around the continent . Most of the press , that so far had repeated the Montevidean libels , turned instead to support Rosas . Brazilian newspapers such as " O Brado de Amazonas " and " O Sentinella da Monarchia " referred to Rosas as a great South American hero . Francisco Antonio Pinto , former president of Chile , declared that the Chilean people was ashamed of the presence in Chile of a pair of newspapers that supported the Anglo - French cause . The president of Bolivia José Ballivián , who was so far against Rosas , instructed his diplomat Manuel Rodríguez to congratulate Rosas for the action of Obligado and protest " with reason of the disloyal and unjust Anglo - French intervention in the Río de la Plata against the rights and interests of the Americas " . José de San Martín wrote a supporting letter to Rosas and , despite his old age , offered his military assistance . He also wrote a letter to the British " Morning Chronicle " , explaining that a military occupation of Buenos Aires by Anglo - French forces would be nearly impossible . The convoy resumed their navigation after the repairs , but with only 52 of the original 90 commercial ships , as the others returned to Montevideo . Mansilla made new attacks at the batteries in Tonelero and Acevedo , but the ships were not damaged very much . By moving to the east side of the river , they could fire against the batteries and destroy them from a safe distance . Mansilla made a more effective resistance at San Lorenzo , in the same site where José de San Martín fought the battle of San Lorenzo . The batteries here were hidden , and attacked the Anglo - French navy by surprise . Many merchant ships collided with others , and the steam ships fired for more than four hours . According to the British report , all ships received shots during the engagement . The " Fulton " arrived to Asunción , with the intention to recognize the independence of Paraguay , recruit them against Rosas , and sign a treaty of commerce and friendship . Carlos Antonio López did not agree to the British terms , he expected recognition first , war later , and a treaty for the end ; not all things at once . The commerce failed , as Corrientes and Paraguay were not as wealthy as the Anglo - French expected , and they returned with most of their products . The return seemed difficult , as many ships were damaged and Mansilla was rearming the north of San Lorenzo , so they requested Montevideo to send reinforcements . The " Philomel " advanced at full speed , not answering fire whenever possible , reaching Montevideo in few days . The British steamboats " Lizard " and " Harpy " moved to reunite with the convoy . Those ships , however , stopped at Quebracho and returned fire , and the " Lizard " was badly damaged as a result . Mansilla prepared a strong defense in Quebracho , against the returning convoy . He did not prepare chains to close the river this time , as the ships would be moving downstream , rather than upstream . This new attack was very successful . The Argentine cannons attacked the enemy ships at will , and the merchant ships attempted in vain to pass behind the warships . Two merchant ships were sunk , and others had to throw their cargo in the river to reduce their weight . The steam boats were the focus of the attack , the " Harpy " was disabled and the " Gorgon " suffered great damages . After three hours of fire , the ships escaped as they could , and four damaged merchant ships were set on fire to avoid Argentine capture . End of the conflict . After the failure of the expedition to the Parana , Ouseley wrote to his government requesting 10 , 000 British soldiers , 10 , 000 French soldiers and an open declaration of war to conclude the conflict . However , unknown to him , Tomás Samuel Hood was already navigating to Buenos Aires with the opposite instructions from the allied countries : negotiate an end to the hostilities , at whatever price Rosas demanded . The repercussion of the battle of Vuelta de Obligado modified the international perception of the conflict in La Plata . Hood had also promoted an end to the conflicts because Rosas had suspended the payment of the Argentine foreign debt to Britain as long as Britain maintained the blockade , and a long conflict would harm the finances of the Baring Brothers bank . Besides , there was an ongoing political scandal in Britain , as " The Times " had published Ouseley was favoring a personal business with the blockade . Hood arranged with Rosas the conditions for peace , but Ouseley and Deffaudis refused to obey it . Deffaudis argued that he had no instructions from Paris to seek a peaceful resolution , and Ouseley that he had to work together with Deffaudis . Hood returned to Britain with the proposal negotiated with Rosas . The British administration was inclined to leave the conflict , but it was reluctant to accept the Hood bases , as they would mean a capitulation . Britain and France sent two new diplomats , John Hobart Caradoc and Alexandre Florian Joseph Colonna , who would pretend to agree with the Hood bases but surreptitiously change the terms . Rosas realized the trick , and did not accept their new proposal . They moved then to Uruguay , and negotiated an armistice with Oribe . Oribe accepted the British terms , but Rivera did not . This broke the alliance : the British diplomat was convinced that the terms were reasonable , and decided to go on with his original mission of ending the conflict . The French one declared instead that they would keep the blockade , even if it meant doing so alone . Rosas received new diplomats , Henry Southern and Lepredour , but refused to have an interview with them before being aware of their intentions . Rosas wanted them to agree to the Hood bases , and would not accept anything else . The Arana - Southern Treaty with Britain was finally signed on March 3 , 1849 , in strict conformity with the terms negotiated with Hood . Britain would return the stolen ships , the Martín García island , remove their troops from Uruguay , accept the Argentine sovereignty over its internal waters , and condition the whole treaty to the approval of Oribe . Finally , the British navy would make a 21 - gun salute to the flag of Argentina . Negotiations with France took a longer time . There was a strong nationalism in France by that time , and a second defeat with Argentina would hurt the national pride . The parliament was divided in two proposals : to send Lepredour with a very powerful navy , to make a treaty favourable to the French terms by intimidating Rosas , or to openly declare war . The first proposal was accepted by 338 votes over 300 . Rosas refused to negotiate unless the threatening navy was removed from Uruguay , and refused to acknowledge Lepredour as a diplomat . Lepredour made up an excuse for the navy , and negotiated for nearly five months . Rosas finally agreed on August 31 , 1850 , to a pair of small concessions that did not actually modify the important points of the treaty : Rosas would remove the Argentine troops from Montevideo at the same time that France removed theirs , but keeping a portion of them during the first months of Oribe ' s rule to prevent anarchy ; and Argentina would refer to Oribe in the document as " President of the Republic " while France would do so as " Brigadier General " . Before leaving the city , the French vessel transporting Lepredour would also make a 21 - gun salute to the flag of Argentina . Historical perspectives . The historical significance of the conflict is disputed between Argentine historians . Revisionist authors consider it a key event in the history of Argentina , next to the Argentine War of Independence , while traditional historians disagree . A recent example of this conflicting viewpoints took place in November 18 , 2010 , before the first celebration of the National Sovereignty day ( in commemoration of the main battle of the conflict , the Battle of Vuelta de Obligado ) as a national holiday . The newspaper La Nación interviewed revisionist historian Pacho O ' Donnell , traditional historian Luis Alberto Romero and British historian David Rock . Luis Alberto Romero considers that the importance of the battle is overrated because it was a defeat : the Anglo - French navy destroyed the artilleries and proceed to the north , as they wanted to do . The end of the blockade in favourable terms to the Confederation was more the result of a change of policy by the Foreign Office after the appointment of Lord Palmerston than a success of Rosas ' diplomacy . Pacho O ' Donnell considers that , even though the navy could force its way , it ultimately failed in its main purposes : they could not turn the Argentine mesopotamia into a new country , nor gain total control of the Parana River , nor establish their presence in the zone . David Rock concurs , but considers that it is exaggerated to treat the battle as an epic . He points that the number of casualties in it may be high in the context of the military history of Argentina , but not on a global scale , as it was nowhere near the 1916 Battle of the Somme , with more than 60 , 000 deaths in just half an hour . Pacho O ' Donnell considers as well that traditional historiography had concealed the battle of Vuelta de Obligado . Romero considers instead that , despite not giving it the higher importance , the battle is properly referenced at all the books about the time period .