Late in 1864 the Brazilian forces marched into Uruguay and joined Flores. Lopez promptly retaliated by seizing a Brazilian steamer which was passing Asuncion on its way to Matto Grosso and followed up this aggression by an invasion of the latter province. His forces quickly reduced the towns on the banks of the Paraguay as far as steamers could penetrate. It was impossible to send reinforcements overland from Rio; Brazil's counter-attack must be delivered from the south. The empire was unprepared, but its troops poured into Uruguay and Rio Grande as fast as they could be mobilised. The anti-Flores party were crushed by the siege and capture of Paysandu late in 1864. The Argentine government under Mitre proclaimed its neutrality. Lopez was flushed with his easy success in Matto Grosso. The forces he had on foot overwhelmingly outnumbered those of the Brazilians in Uruguay and Rio Grande. He wished to strike the latter before they could be re-enforced, overrun Rio Grande, and, as master of one of Brazil's most valuable provinces, dictate terms. To reach the Brazilians it was necessary to cross the Argentine province of Corrientes. He asked for permission to do so and Mitre refused. Notwithstanding the risk involved, he promptly decided to finish up both Argentine and Brazil at the same time. Sending his troops across the Parana he virtually annexed Corrientes and declared war on Buenos Aires. Lopez destined twenty-five thousand men for the invasion of Corrientes and the conquest of the Lower Uruguay valley, but the difficulties of getting so large an army across the river and ready for an advance into a hostile country were unexpectedly great. The gauchos of Corrientes, trained for generations in civil wars, quickly assembled to oppose the Paraguayans. Meanwhile, a Brazilian fleet came up; and, on June 2, 1865, at Riachuelo, decisively defeated the Paraguayan naval forces. Lopez thereby lost all hope of commanding the river. The communications of his army in Corrientes might be cut off at any time and an advance became impossible. The battle of Riachuelo threw Paraguay on the defensive and made Lopez's great plan of carrying the war to the Uruguay impracticable. [Illustration: FRANCISCO SOLANO LOPEZ. [From a photograph taken in 1849.]]