Battle of Tuyutí
The invasion of Paraguay followed the course of the Río Paraguay, from the Paso de la Patria. From April 1866 to July 1868, military operations concentrated in the confluence of the rivers Paraguay and Paraná, where the Paraguayans located their main fortifications. For more than two years, the advance of the invaders was blocked, despite initial Triple Alliance victories.
The first stronghold taken was Itapiru. After the battles of the Paso de la Patria and of the Estero Bellaco, the allied forces camped on swamps of Tuyutí, where they were attacked. The first battle of Tuyutí, won by the allies on May 24, 1866, was the biggest pitched battle in the history of South America.
The battle of Tuyutí
Due to health reasons, in July 1866, Osório passed the command of the First Corps of the Brazilian army to General Polidoro da Fonseca Quintanilha Jordão. At the same time, the Second Corps—10,000 men—arrived at the theater of operations, brought from Rio Grande Do Sul by the baron of Porto Alegre.
To open the way to Humaitá, the biggest Paraguayan stronghold, Mitre attacked the batteries of Curuzu and Curupaity. Curuzu was taken by surprise by the baron of Porto Alegre, but Curupaity resisted the 20,000 Argentines and Brazilians, led by Mitre and Porto Alegre, with support of the squadron of admiral Tamandaré. This failure (5,000 men were lost in a few hours) created a command crisis and stopped the advance of the allies.
During this phase of the war, many Brazilian servicemen distinguished themselves, amongst them, the heroes of Tuyutí: General José Luís Mena Barreto; Brigadier General Antônio de Sampaio, protector of the infantry weapons of the Brazilian Army; Lieutenant Colonel Emílio Luís Mallet, head of the artillery; and even Osório, head of the cavalry. In addition, Lieutenant Colonel João Carlos of Vilagrã Cabrita, head of weapons of engineering, died in Itapiru.
Assigned on October 10, 1866 to command the Brazilian forces, Marshal Luís Alves de Lima e Silva, Marquis and, later, Duke of Caxias, arrived in Paraguay in November, finding the Brazilian army practically paralyzed. The contingent of Argentines and Uruguayans, devastated by disease, were cut off from the rest of the allied army. Mitre and Flores returned to their respective countries due to questions of internal politics. Tamandaré was replaced in command by the Admiral Joaquim José Inácio, future Viscount of Inhaúma. Osório organized a 5,000-strong third Corps of the Brazilian army in Rio Grande do Sul. In Mitre's absence, Caxias assumed the general command and restructured the army.