By late 1864, Paraguay had scored a series of victories in the war, but on 11 June 1865, its naval defeat by the Brazilians on the Paraná River began to turn the tide in favor of the allies.

[3] Paraguay's fleet was a fraction of the size of Brazil's, even before the battle, and arrived at the Fortress of Humaitá on the morning of June 9.

The Paraguayan president Francisco Solano López prepared to attack the ships supporting allied land troops at Riachuelo.

[4] The Paraguayans had planned a surprise attack before sunrise since they were fully aware that most Brazilian troops would offboard their steamers to sleep on land, and they would leave only a small garrison of men to guard and watch their fleet.

The original plan had been that under the dark of the night, the Paraguayan steamers would sneak up to the docked Brazilian vessels and board them outright.

[4][5] No confrontation other than the one carried out by the boarding party had been planned, and the Paraguayan steamers were there only to provide cover from the inland battling forces.

[1] The fleet arrived at Corrientes after sunrise, but because of a dense fog, the plan was still executable since most, if not all, Brazilian forces were still on land.

[1] Once out of range, they turned upstream and anchored the barges, which formed a line in a very narrow part of the river.

Barroso famously rallied his fleet signalling "Brazil expects that every man will do his duty", paraphrasing Horatio Nelson.

Beberibe and Araguari pursued the Paraguayans and heavily damaged Tacuary and Pirabebé, but nightfall prevented the sinking of those ships.

[10] President López ordered Major José María Bruguez with his batteries to quickly move inland to the south to wait for and attack the passing Brazilian fleet.

[9] Also, under orders from López, one month after the battle, the Yporá returned to the scene and, again under the cover of night and using stealth to avoid alarming another Brazilian steamer nearby, boarded the remains of the Jequitinhonha and stole one of its cannons.