Origins

Although many historians commonly trace the coming of war through the 1862s, some roots of it were present as early as the colonial period.

Portugal and Spain have disputed La Plata Region (Uruguay, Northwest of actual Argentina and South Brazil) since the animosity of the two metropolis emerged in Europe in the XVII century.

When the colonial countries conquered their independence in the beginning of the XIX century, they inherited the boundary conflicts from those two nations.

The struggle for hegemony involved foremost the government of Buenos Aires ( Argentina's capital) and the Brazilian Empire. After many boundary skirmishes, both went to war over the disputed Uruguay. In 1827, a combined Argentine-Uruguayan force defeated the Imperial Army on the Battle of Passo do Rosário (the Argentinean called it Ituzaingó). Nevertheless, on the sea the Imperial Navy imposed its predominance over the foes. Thanks to this dilemma, Uruguay obtained the independence in 1828.

Brazilian Troops on the way to Uruguay in the beggining of the XIX century

Picture by Jean Baptiste Debret

Buenos Aires also had its own problems with the surroundings Argentinean Provinces. In fact, they have never accepted Buenos Aires' hegemony in the confederation. They waged war on each other several times. The only thing that could keep them together was their common hate for the Brazilian Empire.

Threatened by Buenos Aires pretensions of incorporating it to Argentina, Paraguay conquered its independence after the Battle of Tacuarí in 1811. Nevertheless, Paraguay would not be free of concealed menaces for almost fifty years. Brazilian Empire also had contentions against Paraguay over the Apa River region.

Finally, there was Uruguay, that had to play a dangerous game to keep its independence, surrounded it was by the two South America giants: Brazil and Argentina.

In such web of contradictory interests, caution should be a virtue, mainly by the two smallest countries of the region. Carlos Antonio López, Solano López' father, was aware of it. He had decided for a non-interventionist policy, even when Brazil called for his aid to back an alliance against the Argentinean dictator Juan Manuel Rosas in 1852.

When Solano López assumed power after his father's death in 1862, he came closest to Uruguay's Blanco faction. When Brazil intervened in the habitual strife between the Blanco and Colorado wings in benefit of the last one on August 1864, López assumed it as a threat to Paraguay's interests. He sent an advice to Rio de Janeiro's government  not to break the tenuous balance of Uruguay's internal policy.

On October 16, 1864 the Imperial Fleet blocks Montevideo (Uruguay's capital) and 4,000 troops cross Brazil-Uruguay boundary. A casus belli for Solano Lopez.

The Opponents