The French blockade of 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 did not land ground forces, but instead took advantage of the Uruguayan Civil War and the Argentine Civil Wars, supporting Fructuoso Rivera and Juan Antonio Lavalleja 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 Supreme Protector of the Peru–Bolivian Confederation, Andrés de Santa Cruz, supported free trade with Europeans, as well as the Unitarians, whereas Rosas was leader of the Federales. Rosas would later declare war against the Confederation for harboring Unitarian leaders. The war did not develop favorably for Argentina, and the French consul Aimé Roger went to Buenos Aires to request the capitulation of Argentina. He demanded that a pair of French citizens were released from prison. Those citizens 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 favoured nation". This designation would provide commercial privileges, similar to those granted by Bernardino Rivadavia to Britain. Although the requests were light, Rosas believed 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 out 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 the blockade being short and Rosas being 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 neighboring countries allied with them, 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 pledging his full support. San Martín repudiated the Unitarians that allied themselves with a foreign country against their own nation, saying that "not even death can expunge such felony". He also offered his services to Rosas in the war, who declined due to San Martín being over 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 have doubts about continuing a conflict that they had counted on being 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 Rosas to have nationalized a conflict that involved just Buenos Aires, and proposed to the French that Santa Fe, Córdoba, Entre Ríos and Corrientes secede, 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.