Even after the disastrous defeat at the Battle of Riachuelo, the Paraguayan soldiers continued advancing overland towards Rio Grande do Sul, under the command of lieutenant colonel Antonio de la Cruz Estigarribia.

A few days earlier, on June 8, the Paraguayan column, organized by major Pedro Duarte and numbering about 10,000 men, was in the Argentine village of Santo Tomé, close to 8 km (5.0 mi) from the border with Brazil.

Colonel Antônio Fernandes Lima, head of the Brazilian forces in the frontier, was warned about Paraguayan movements on the other side of the river, but did not believe in the speed of their preparations.

[1] At 10 in the morning the Paraguayans began the attack on São Borja, encountering weak resistance from lancers commanded by colonel Ferreira Guimarães.

1,400 Paraguayans from the column of captain Diogo Alvarenga were bayoneted by two hundred infantrymen commanded by Floriano Peixoto, who would later become President of Brazil.