threaten the free navigation of the River Plate basin, which makes up one-fourth of the South American continent's surface. As a result, Brazil reinforced its fleet in the River Plate. Argentina began to acquire war materiel and to buy warships. The American minister in Rio de Janeiro also thought that Argentina was behind an unsuccessful attempt to topple President Jovellanos of Paraguay, who, the Argentines said, was a Brazilian puppet. In April 1874, Uruguay, the junior partner of the alliance, negotiated a separate treaty of friendship with Paraguay, under the auspices of Brazil. Argentina broke off diplomatic relations with Uruguay. The American minister thought that if war broke out between Argentina and Brazil, Uruguay would be on Brazil's side and that the Argentine provinces of Corrientes and Entre Ríos, where there was considerable opposition to the government in Buenos Aires and were virtually self-governing, would break away. He would not be surprised if, under Brazilian influence, there was formed "a new Riverine Republic... [which] would be composed of Uruguay, Entre Rios, Corrientes and Paraguay". In 1875, Argentina started fortifying Martín García Island again, which the US saw as a full treaty violation, threatening free navigation: "The problem was compounded by the appearance of two new Argentine ironclads, which caused further Brazilian apprehension and curiosity". In 1875, Emperor Dom Pedro II of Brazil decided to try to prevent a war with Argentina by taking up the matter with Tejedor himself. Matters seemed to be progressing well until Tejedor suddenly left Brazil. It appeared that Tejedor had been negotiating Argentina's own unilateral treaty with Paraguay, taking advantage of the fact that Paraguayan envoy Jaime Sosa was in Rio de Janeiro at the time. They signed the Tejedor-Sosa treaty, which called for the territory of Villa Occidental to be given to Argentina. The treaty was quickly rejected by the Paraguayan government. It seems that Tejedor and Sosa expected that outcome, but that Tejedor signed the treaty anyway to defy Brazil. In the end Brazil and Argentina resolved their differences because "[they] had both learned from past experience that even a successful war would end in national financial disaster". In December 1876 they signed an agreement by which the island of Cerrito was ceded to Argentina, the Chaco south of the Pilcomayo River went to Argentina, the northern Chaco was left to Paraguay, and the Central Chaco was to be arbitrated between Argentina and