Faced with the advance of liberal forces backed by the empire, the conservative Paraguayan government of Marshal Francisco Solano López anticipated that Brazil and Argentina would try to incite a civil war in Paraguay.

The Paraguayan invasion of Mato Grosso was a success, but the Partido Blanco government in Uruguay was defeated and General Flores assumed the presidency.

The attack would be launched after the expected delivery date of the declaration of war but before the news arrived back in Paraguayan territory to give the disorganized, under-equipped Argentine army no time to react.

Robles loaded as many soldiers as he could; the small Paraguayan squadron returned to Corrientes at dawn on 14 April, taking possession of the city square without any resistance.

[6] Paraguayan control of the square was irreversible in principle, so Governor Lagraña, his closest collaborators, and security groups moved to rural areas to avoid being taken prisoner.

Before he withdrew, Lagraña ruled that every citizen of Corrientes between sixteen and seventy years of age was required to enlist and fight the occupation forces.

Local political action was carried out by Cáceres, but the triumvirate was limited to endorsing the activities of the Paraguayan commissioners José Berges, Miguel Haedo and Juan Bautista Urdapilleta in commercial matters and relations with Paraguay.

The Federalist Party leaders in the capital initially supported the Paraguayan occupation as allies in their attempt to recover the political dominance they had lost at the end of 1861 after the Battle of Pavón and the Corrientes revolution.

The presence on the governor's side of Cáceres, who (despite his ambiguous record) was considered to belong to the Federalist Party, cooled federal enthusiasm for the invaders and deprived them of provincial support.

As some of them refused to receive Paraguayan paper money, they were exterminated with sabers and bullets in broad daylight.The population of the big cities denounced the invasion, which they saw as unjustified and treacherous.

Respecting his previous commitments, governor and former president Justo José de Urquiza assembled a provincial army of 8,000 men and moved it to the northern border of the province.

When they reached Corrientes in July 1865, soldiers who had believed that they were going to fight on the Paraguayan side rose up in the Disbandment of Basualdo [es] and deserted en masse.

Mitre gathered the available troops in Buenos Aires, Rosario, and San Nicolás de los Arroyos, and moved a strong division north aboard the war fleet.

In late April, the Paraguayan army slowly advanced south along the Paraná River and seized Bella Vista, Empedrado, Santa Lucía, and Goya.

Battles at Caá Catí and Naranjito consolidated the route along the rivers, since the center and south of Corrientes Province remained under government control with a new capital in San Roque.

The four battalions were commanded by Juan Bautista Charlone, Ignacio Rivas, and Adolfo Orma [es]; Manuel Rosetti, with an artillery squad, was also part of the division.

The Paraguayans, commanded by Major José del Rosario Martínez, withdrew towards Empedrado to reorganize and receive reinforcements; a large division was advancing from Paso de la Patria toward the capital.

Although this decision made it difficult for Paunero's men to resist the expected counterattack, their presence in the south of the province prevented the Paraguayan division which had occupied Goya and Santa Lucía from advancing to support the Uruguay River.

A battery commanded by Major José María Bruguez, hidden in the forests of ravines north of the mouth of the Riachuelo, was to bombard ships fleeing the ambush.

The Brazilian fleet did not take advantage of the victory; the next day, it set off downstream towards the outskirts of Emedrado after preventing communications between Paraguay and the Atlantic Ocean.

While the city of Corrientes was occupied, a garrison of 12,000 men commanded by Lieutenant Colonel Antonio de la Cruz Estigarribia headed east of the province to attack Brazilian territory on the Uruguay River.

President Mitre appointed General Urquiza, Governor of Entre Ríos, commander of the vanguard division with the mission of confronting the Uruguayan column.

Paunero's 3,600 men began an accelerated march through estuaries and rivers, rapidly crossing the south of the province of Entre Ríos, to join Flores.

Among the prisoners, Flores found several dozen Uruguayan soldiers (supporters of the Blanco Party who had taken refuge in Paraguay) and federalist Argentines who did not recognize Mitre's national authority.

Presidents Bartolomé Mitre and Venancio Flores were already present, as well as Brazilian military leaders such as the Marquis of Tamandaré and Lieutenant General Manuel Marques de Sousa.

The head of the Paraguayan division of the allied army wrote to Estigarribia, rejecting his charge of treason and accusing López of betraying his country by oppressing his people.

Passing La Cruz, the vanguard (commanded by Colonel Fernández Reguera) discovered a Paraguayan division with three artillery batteries leading a herd of 30,000 cattle from Corrientes to their country.

[20] General Nicanor Cáceres reported in August that "the towns of San Roque and Bella Vista that the invaders have occupied for more than two months (...) as well as all the fields through which they have crossed are spoils capable of encouraging the most indifferent".

At the end of the year, the allied army in the Ensenadas (or Ensenaditas) camp a few kilometers north of Corrientes – near present-day Paso de la Patria – reached 50,000 men.

They advanced a few kilometers to the Arroyo Pehuajó, where General Conesa was waiting in ambush; before the attack he addressed his troops, who burst into loud cheers (betraying their presence to the Paraguayans).