After crossing the Paraná River from Argentina and landing in Paraguay, they had a long march across country studded with lagoons and carrizal (reed beds growing in marsh).

Paraguayan leader Francisco Solano López moved his headquarters to Paso Pucu, where he dug trenches in the passes from Gomez to Rojas.

Colonel José Eduvigis Díaz, with 6,000 infantry and 1,000 cavalry, attacked the Allied center, general Flores' Vanguard Division.

Colonel Hilario Marcó, with 7,000 men and 48 cannons, formed the reserve at Estero Rojas[1]: 53–54 The attack began in the center, where the Uruguayans were forced back along with some Brazilian Volunteer battalions.

On the left of the Allied encampment, lieutenant-colonel Émile Mallet had ordered a large moat to stealthily be dug in front of his artillery pieces, which were thirty La Hitte cannons, according to one source.

The Paraguayans tried to circle the artillery, avoiding the incoming fire, but encountered Antônio Sampaio's 3rd Infantry Division.

Osório reinforced the Brazilian lines with various units, finally committing the 2nd Cavalry Division, commanded by general João Manuel Mena Barreto.

"[7] As result of the battle, each side's losses were as follows: There have been contradictory reports about the casualty numbers and debates over the true values.

"The Allies buried some of their own dead, but they heaped up the Paraguayan corpses in alternate layers with wood, in piles of 50 to 100, and burnt them.

These months of static warfare were filled with small-scale skirmishes and sporadic fire, being known in the Allied forces as the "Tuyutí Black Lines[16]".

The Brazilian Army's Patrons of the Infantry (Antônio de Sampaio), Cavalry (Manuel Luís Osório) and Artillery (Émile Mallet) fought in Tuyutí.

At the head of the 3rd Division of the Imperial Army, dubbed the Ironclad Division, composed of the Arranca-Toco, Vanguardeiro and Treme-Terra battalions, brigadier general Antônio de Sampaio fought in the crossing operations of the Paraná River, in the Battle of Confluência and in the Battle of Estero Bellaco.

At the Battle of Tuyutí (May 24, 1866, ironically the date of his birthday), Sampaio was seriously injured three times by artillery shrapnel, gangrenizing his right thigh, and twice on his back.

Buried in that capital on 8 July 1866, his remains were repatriated in 1869 to Rio de Janeiro, being deposited in the Church of Bom Jesus da Coluna, in the Asilo dos Inválidos da Pátria (Asylum of the Invalides of the Motherland), where they remained until 14 November 1871, when they were transferred again to his homeland in the province of Ceará.