In 1850 the Emperor chose a new Conservative Cabinet, which proved the most efficient the country had known. Its first great act was to abolish the slave trade. The year 1850 is also memorable as that in which the yellow fever began those terrible ravages on the Brazilian coast which have never since entirely ceased. The first epidemic is said to have been the worst which ever visited Rio. Two hundred persons fell sick daily, and the wealthier classes were especially attacked. Among the victims was the great statesman, Bernardo de Vasconcellos, and many deputies, senators, and diplomatic representatives. Congress adjourned in terror. In the earlier epidemics the citizens of Rio were just as susceptible as foreigners. Later, however, they acquired a relative immunity--an immunity which is not shared by Brazilians who have lived in non-infected districts. Brazil and Argentina had agreed in 1828 that Uruguay should be an independent and neutral buffer state between them. But the Buenos Aireans never forgot that for geographical and historical reasons Uruguay naturally belonged to them. Rosas, the Argentine dictator, assisted the Oribe faction, which openly advocated entering the confederation, while the Rio Grande Brazilians who owned much property on the Uruguayan side of the border aided the Rivera faction. To protect the property interests of its citizens and prevent Rosas from conquering Uruguay the Brazilian government quietly made military preparations and formed an alliance with the Rivera party and with Urquiza, the ruler of the province of Entre Rios, to which the dictator of Paraguay and the president of Bolivia gave a passive adhesion. It amounted to a coalition to forestall Rosas's plan of uniting the whole of the old Viceroyalty and the Plate valley under his rule. Brazil was virtually the instigator of a combination of the weaker Spanish-American states against the strongest one. Urquiza crossed the Uruguay, and with the aid of the Brazilian troops made short work of Oribe's army, which was besieging Rivera in Montevideo. Rosas responded with a declaration of war and began collecting a formidable army. Urquiza resolved to carry the war to the gates of Buenos Aires. The allies gathered in camp on the left bank of the Parana, a hundred miles above Rosario, a great army which numbered four thousand Brazilians, eighteen thousand Argentines, mostly from the half-Indian provinces of Entre Rios and Corrientes, and a contingent of Uruguayans. A Brazilian fleet under Admiral Grenfell had penetrated up the Parana and protected their crossing of the great river. On the 17th of December they got safely over the Parana, and out of the low country of Entre Rios on to the dry pampas of the right bank. Thence they marched down on Buenos Aires, where Rosas was awaiting them. On the 3rd of February, 1852, he gave them battle in the suburbs of that city. He was completely defeated and fled to England.