The Invasion of Rio Grande do Sul began on June 10, 1865 when about 7,500 soldiers under the command of General Antonio de la Cruz Estigarribia invaded the village of São Borja near Brazil's border with Argentina.

About 3,000 men commanded by major Pedro Duarte stayed on the other side of the Uruguay River to accompany the advance of the main column.

The Corrientes campaign began when the Paraguayans invaded Argentina in April 1865 after the Argentines denied the passage of López's troops over their territory.

At 10 am, the Paraguayans began the Battle of São Borja, encountering weak resistance from lancers commanded by colonel Ferreira Guimarães.

Estigarribia knew the extreme danger of crossing the Ibicuí River as it was full of currents, the potential loss of the supply carts, cannons and luggage and the possibility of an ambush by colonel Fernandes Lima.

After several stages from the allies, the last being on September 18, with the threat of invasion of the city, in addition to hunger and lack of help from López, Estigarribia surrendered with about 6,000 of his men, thus ending the campaign in Rio Grande do Sul.