As a result, Argentina and Uruguay entered the war, which previously only referred to Paraguay and Brazil, signing with the latter a secret pact that was called "Triple Alliance".

President Bartolomé Mitre, in the midst of a violent demonstration against Paraguay, delivered a famous speech in which he said: "Gentlemen, after the provocation was launched, after the insult perpetrated on our flag by the tyrant of Paraguay, whoever governs you cannot tell you anything other than that the proclamations and demonstrations will be translated into facts, and that in twenty-four hours we will be in the barracks, in fifteen days on the battlefield and in three months in Asunción".Corrientes Governor Manuel Ignacio Lagraña, who remained loyal to Mitre, had left the city shortly before Robles arrived; for that reason, the Paraguayans set up an assembly on April 19, which appointed the regency a triumvirate formed by Teodoro Gauna, Víctor Silvero and Sinforoso Cáceres.

While the governor of Corrientes, housed in the city of San Roque, brought together 3,500 unarmed civilians and military, joined later by 1,500 veteran soldiers of the Argentine Army.

As Robles marched south, facing the resistance of small Argentinian armed groups that had no chance of arresting him, occupying in sequence Bella Vista, Empedrado, Santa Lucia and Goya, 250 kilometers to the east, a second column of 12,000 men, commanded by Lt. Col. Antonio de la Cruz Estigarribia, crossed the Paraná river near Encarnación to go south along the right bank of the Uruguay River.

On May 25, an Argentine squad composed of 725 soldiers, commanded by General Wenceslao Paunero, unexpectedly attacked Corrientes; after a tough house-to-house battle, the defeated Paraguayans withdrew from the city to neighboring Empedrado, leaving more than 400 dead behind.