The Battle of Pavón, a key battle of the Argentine Civil Wars, was fought in Pavón, Santa Fé Province, Argentina on 17 September 1861 between the Army of the State of Buenos Aires (commanded by Bartolomé Mitre) and the Army of Republic of the Argentine Confederation (commanded by Justo José de Urquiza).

President Santiago Derqui issued a decree invalidating the elections in Buenos Aires and established a new date for a rerun.

But the Buenos Aires authorities rebelled against the national government and declared the Pact of San José de Flores null.

Urquiza tried, until the last moment, to preserve the peace and declined to take the initiative against the porteño army as it was the request of his colonels Ricardo López Jordán and Prudencio Arnold.

In sum, the federalist army had about 17,000 men, where 8,000 came from the center region and 9,000 from Entre Ríos, Corrientes, Buenos Aires and Santa Fé.

But Urquiza's artillery started combat, opening great gaps in the porteño infantry, easy targets due to their colorful uniforms.

Combat lasted only two hours, during which the federalist left wing under colonel major Juan Saá, with the Santa Fé and renegade porteño troops of Ricardo López Jordán, completely vanquished the porteño First Cavalry, under general and former Uruguayan president Venancio Flores, chasing them past Arroyo del Medio (a creek forming the border between Buenos Aires and Santa Fe provinces).

The porteño Second Cavalry, under the command of veteran general Manuel Hornos, offered more resistance; but it had to retreat, leaving behind most of their heavier weapons and supplies plus many prisoners.

The federalist center, instead, composed by untrained militia from the central regions of the country, was forced to retreat by the better trained and equipped porteño infantry battalions.

Seeing the center's collapse, Urquiza abandoned the field of battle without adding the 4,000 men from Entre Ríos that he had maintained in reserve, and marched to Rosario, then followed to San Lorenzo and Las Barrancas.

Urquiza's unexpected decision left the field open to the porteño army, which had retreated to San Nicolás de los Arroyos.