After López's declaration of war on Brazil, Barrios was appointed supreme commander of the two army units of 5,000 men that invaded the Brazilian province of Mato Grosso from Alto Paraguay on separate routes from mid-December 1864.

[2][3] The Mato Grosso Campaign by Paraguayan forces was of little strategic use to the country, but it brought in significant amounts of arms and ammunition, as well as tens of thousands of head of cattle and other loot.

Barrios remained in this position, even after he was recalled to active military service after an army from the Triple Alliance countries of Argentina, Brazil and Uruguay invaded Paraguayan territory.

[5] On this battle, conceived by López as a large-scale surprise attack designed to smash the Allied army and drive it back out of Paraguay, ended in a heavy defeat for his force.

However, the relations between the two men fell off after the Imperial Brazilian Navy had succeeded in February 1868 in overcoming the Paraguayan river barriers at the Humaitá fortress and shelling the capital Asunción for the first time.