Causes of friction still remained, but they did not develop into open quarrels, for Mitre was content to leave Urquiza in his province of Entre Rios, and the other administrators (caudillos) in their several governments, a large measure of autonomy, trusting that the position and growing commercial importance of Buenos Aires would inevitably tend to make the federal capital the real centre of power of the republic. In 1865 the Argentines were forced into war with Paraguay through the overbearing attitude of the president Francisco Solano Lopez.
In 1864 the ambitious dictator of Paraguay, Francisco Solano Lopez, without previous declaration of war, captured a Brazilian vessel in the Paraguay, and rapidly followed up this outrage by an armed invasion of the provinces of Matto Grosso and Rio Grande in Brazil, and that of Corrientes in the Argentine Republic. A triple alliance of the invaded states with Uruguay ensued, and the tide of war was soon turned from being an offensive one on the part of Paraguay to a defensive struggle within that republic against the superior number of the allies.
On Francia's death, in 1840, the chief power passed to his nephew, Carlos Antonio Lopez, who in 1862 was succeeded by his son Francisco Solano Lopez.
From Ordaz up to recent times the Orinoco has been the scene of many voyages of discovery, including those in quest of El Dorado, and some scientific surveys have been made, especially among its upper waters, by Jose Solano and Diaz de la Fuente of the Spanish boundary line commission of Yturriaga and Solano (1757-1763), Humboldt (1800) and Michelena y Rojas (1855-1857).