On 16 April 1866, Allied troops under the command of General Osório, crossed the River Paraguay and attacked Fort Itapirú and Paso de la Patria.

Flores' forces included: the Brazilian 5th, 7th, 3rd, 16th, 21st and 38th Voluntarios da Patria, the Uruguayan 24 de Abril, Florida, Independencia and Libertad, and the Argentinian Rosario.

General Flores and his vanguard, commanded by the Spanish mercenary León de Palleja, stationed in Estero Bellaco, was attacked on 2 May by a Paraguayan force of 6,000 men with four pieces of artillery.

Combat of 2 May 1866 in the Estero Bellaco Sud: attack by the vanguard of the Allied Army by the division of Paraguayan Lieutenant Colonel José E. Díaz, later General.

At twelve o'clock, when the allies were surrendering to devour the ranch, the Paraguayans burst through the three passes of the Estero, overwhelming the outposts of the vanguard.

Episode of the 1st Line Cavalry of the Argentine Army in the Combat of Estero Bellaco (reduced reproduction of a plate drawn by the distinguished artist Francisco Fortuny).

Instead of ordering the withdrawal immediately, since the objective of the operation had already been fulfilled, he engaged in a reckless pursuit to crash into the bulk of the allied army.

On the other side of the Estero, Díaz made an enveloping movement of the Brazilian troops fail, attempted by Paso Sidra, repelled them twice with the bayonet, forcing them to flee.

The Paraguayans were able to get 4 pieces of artillery and several carts full of modern weapons that served to alleviate the shortage of material that was beginning to be felt in Solano López's camp.