French blockade of the Río de la Plata The French blockade to the Río de la Plata was a two - year - long naval blockade imposed by France on the Argentine Confederation ruled by Juan Manuel de Rosas . It closed Buenos Aires to naval commerce . It was imposed in 1838 to support the Peru – Bolivian Confederation in the War of the Confederation , but continued after the end of the war . France didn ' t land ground forces , but instead took advantage of the Uruguayan Civil War and the Argentine Civil Wars , supporting Fructuoso Rivera and Juan Lavalle against Manuel Oribe and Rosas . After two years without the expected results , France signed the Mackau - Arana treaty with the Argentine Confederation , ending the hostilities . Development . Start of the blockade . The Peru – Bolivian Confederation declared the War of the Confederation over Argentina and Chile . His " Supreme Protector " Andrés de Santa Cruz supported the European interests in South America , as well as the Unitarians , whereas Rosas and the Chilean Diego Portales did not . As a result , France gave full support to Santa Cruz in this war . Britain also supported Santa Cruz , but only by diplomatic meanings . Trusting in the military power at his disposal , Santa Cruz declared war to both countries at the same time . Initially , the Peru - Bolivian forces had the advantage , and captured and executed Portales . The war did not develop favorably for Argentina in the north , and the French consul Aimé Roger moved to Buenos Aires to request the capitulation of Argentina . He demanded that a pair of French citizens were released from prison . Those were César Hipólito Bacle , who had sold Argentine cartography to Bolivia , and Pedro Lavié , who had stolen from a regiment in Dolores . It was also demanded that another pair were exempted from military service , and that France received the condition of " " most favored nation " " . This designation would provide commercial privileges , similar to those granted by Bernardino Rivadavia to Britain . Although the requests were light , Rosas considered that they would only provide precedent for further French interference in the internal affairs of Argentina , and refused to comply . As a result , France started a naval blockade over Buenos Aires . The French admiral Louis François Jean Leblanc started it on March 28 , 1838 . Rosas took advantage of the British interests in the zone . The minister Manuel Moreno pointed to the Foreign Office that the commerce between Argentina and Britain was being harmed by the French blockade , and that it would be a mistake for Britain to support it . France underestimated the chance of having such problems with its European ally , counting on that the blockade would be short and Rosas would be forced to leave the government in a short time . They judged that the people would seize the opportunity to stand against Rosas , but underestimated his popularity . With the nation being threatened by both two European powers and two neighbour countries allied with them , the patriotic loyalty increased to the point that even some notable Unitarians who had fled to Montevideo returned to the country to offer their military help , such as Soler , Lamadrid and Espinosa . José de San Martín , who was living in France , wrote a letter to Rosas giving him his full support . San Martín repudiated the Unitarians that allied themselves with a foreign country against their own nation , saying that " " such felony , not even the tomb can make it disappear " " . He also offered his service to Rosas in the war , who declined it because San Martín was more than sixty years old . Attempts of secession . Things became more complicated for France as time passed : Andrés Santa Cruz was weakening , the strategy employed by Moreno was bearing fruit , and the French themselves started to doubt about keeping a conflict that they had counted to be quite short . Even more , Britain would not allow the French to deploy troops , as they did not want a European competitor gaining territorial strength in the zone . Domingo Cullen , governor of Santa Fe replacing the ill López , considered that Rosas had nationalized a conflict that involved just Buenos Aires , and proposed the French to secede Santa Fe , Córdoba , Entre Ríos and Corrientes , making a new country that would obey them , if the naval blockade was spared to this new country . To complete the pincer movement , France would need another army attacking Rosas from the east . 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 . France also took control of the strategic island Martín García . The alliance between Cullen and Rivera did not take place , as Juan Pablo López , brother of Estanislao López , defeated Cullen and drove him away from the province . Collen escaped to Córdoba and then to Santiago del Estero , but the governor Felipe Ibarra wanted to stay with good relations with Rosas , so he captured Cullen for him . In the north , Andrés Santa Cruz was defeated by the Chilean army at the Battle of Yungay , and the Peru – Bolivian Confederation ceased to exist . Now Rosas was free to focus all his attention towards the French blockade and Rivera . Lavalle joins the war . Rivera was urged by France to take military action against Rosas , but he was reluclant to do so , considering that the French underestimated his strength , even more after Santa Cruz ' s defeat . In paper , Rivera was supposed to cross the Parana with 600 men , Entre Ríos would immediately join the resistance against Rosas and the army would grow to 6 . 000 men , the same would happen in Santa Fe and the size of the army would be duplicated , and then it would attack Buenos Aires with support of the French navy , whose population would revolt against Rosas . The French and Unitarians believed in this scenario , but Rivera was aware that the popular support to Rosas was genuine , so sending a small army to Buenos Aires would be doomed to failure . As Rivera did not take action , they elected Juan Lavalle to lead the attack , who shared the unitariam optimism on the fate of a small army . He requested not to share command with Rivera , and as a result they led both their own armies . His imminent attack was backed up by conspiracies in Buenos Aires , led by former members of the May Association . The most notable member of the conspiration was Ramón Maza , son of the former governor Manuel Vicente Maza , who got military support . As Lavalle was delaying , they developed a new plan : Pedro Castelli and Nicolás Granada would make a revolt at Tapalqué , while the military in the city killed Rosas , Manuel Maza assumed government and allowed Lavalle to take the city . The plot was discovered by the Mazorca , but Rosas thought that Manuel Maza was innocent and carried to the plots of his son , so he urged him to leave the country . He could not : Martínez Fontes , one of the military talked into the complot , revealed it in public . Popular commotion was high , and the people took the streets demanding the execution of the people involved with the complot . Ramón Maza was executed , and his father was killed in his office by the Mazorca . Nevertheless , Pedro Castelli attempted to make a rebellion in the countryside . The people did not follow him , and he was executed as well . Ending of the blockade . Stir between Britain and France started to increase . Lord Sandon made critics to France at the British Parliament on March 19 , 1839 . He said that France was attacking a foreign country just because of not signing a treaty , and that it had toppled Oribe from power without being at war with Uruguay . Mr . Lushington added that the French pretensions were unjustified and would have never been enforced against a country with the means to defend itself . Rosas did not wait to be attacked , and ordered Pascual Echagüe to cross the Parana and extend the war into Uruguay . The Uruguayan armies split : Rivera returns to defend Montevideo , and Lavalle moves to Entre Ríos alone . He called all people to stand against Rosas , despite of colour or political ideas , but he found severe resistance , so he moved to Corrientes to join governor Ferré . Ferré defeated López , and Rivera defeated Echagüe , leaving Lavalle a clear path towards Buenos Aires . However , by that point France had given up the trust on the effectiveness of the blockade , as what was thought to be an easy and short conflict was turning into a long war , without clear security of a final victory . The admiral Leblanc was replaced by Dupotet , with new orders to negotiate peace with the Confederation in an honourable way . Those negotiations would include the British ambassador Mandeville . As part of the peace treaty , France requested again the status of " " most favored nation " " of the initial request , but allowing a similar status for the Confederation by France . As a result , France removed financial support to Lavalle . He didn ' t find help at local towns either , and there was strong desertion in his ranks . Buenos Aires was ready to resist his military attack , but the lack of support forced him to give it up and retire from the battlefield , without starting any battle . His army escaped to the north in disorder , and he died in a confusing episode at San Salvador de Jujuy .