In 1863, Flores, a defeated chief, returned from Buenos Aires and set up the standard of revolt in Uruguay. Penetrating as far as the Brazilian border he received assistance, and Aguirre, the Montevidean president, protested. At the same time the latter ruler refused to settle certain claims on behalf of Brazilian citizens which the Rio government had been pressing. The Emperor decided to intervene and help Flores, and thereupon sent a man-of-war up the Uruguay River, which blockaded a port and destroyed Uruguayan public property. Aguirre declared war, and Brazil and Flores in alliance besieged and took the principal towns in western Uruguay. The Argentine received satisfactory assurances and remained neutral. This high-handed adjustment of Uruguayan affairs furnished a pretext to the Paraguayan dictator, Francisco Lopez, to intervene in his turn. Under a line of vigorous dictators who concentrated all the forces of the nation into their own hands, that country had become menacing to the loosely organised Argentine Republic. Lopez even thought he was strong enough to bid defiance to Brazil. The tyrant was, in fact, an impossible neighbour for the two more progressive and civilised powers. For years he had been preparing for war and at the moment was stronger in a military way than either of his bulky neighbours. He hated both Argentines and Brazilians, and his people had been taught to despise the courage of the latter. Though Brazil's intervention in Uruguay was a matter in which he had an interest, a dignified protest would have obtained ample assurances that the latter's independence would be respected, for there is no evidence that the imperial government intended to do anything more than to replace its enemy Aguirre by the friendly Flores. But the arrogant tyrant wanted to draw the world's attention to himself. He appreciated how difficult it would be for Brazil to send an army against him and how much more difficult it would be to maintain one, and he also knew that she was unprepared to undertake a serious war on foreign soil. Without any declaration of war, in the fall of 1864 he seized a Brazilian steamer which was making its regular trip up the Paraguay River to Matto Grosso. The crew were imprisoned, and only the intervention of the American minister saved the lives of the Brazilian minister and his family. This outrage left Brazil no alternative. Lopez followed up the seizure of the boat by an expedition up the Paraguay River against Matto Grosso, and easily conquered the principal southern settlements in that province.