On April 1, 1859, following the assassination of former San Juan Province Governor Nazareno Benavídez by a presumed Buenos Aires agent, the Confederation Congress passed a law by which the President Justo José de Urquiza was obliged to "peacefully reincorporate the dissident province of Buenos Aires", but if that was not possible, he was allowed to use the national army to accomplish that purpose.
The government of Buenos Aires interpreted that law as a formal declaration of war. In May, the state legislature allowed the governor to repel any military aggression with the province's militia. Colonel Bartolomé Mitre, in charge of Buenos Aires troops, was ordered to attack Santa Fe Province, and the navy was sent to blockade Paraná, the capital of the Confederation.
With the imminence of conflict, Brazil, Paraguay, the United States and the United Kingdom tried to prevent it by diplomatic means. The neighboring country of Paraguay sent a young Francisco Solano López as a plenipotentiary minister to intercede in the emergency. However, every attempt at a peaceful resolution of the conflict failed since Buenos Aires demanded Urquiza's resignation as president, which the Confederation refused.
Battle
The army of the Confederation met Buenos Aires forces at Cañada de Cepeda, north of Pergamino. After some tactical movements, both forces clashed on the afternoon of October 23. By dusk, Mitre knew that his forces were defeated and so the bonaerense army retired towards San Nicolás de los Arroyos from where they embarked back to Buenos Aires.
Aftermath
Urquiza did not enter Buenos Aires City but camped at the neighboring town of San José de Flores to carry on with the negotiations. Governor Valentín Alsina was forced by his own allies to leave office because of his intransigent position to the reintegration of Buenos Aires to the Confederation.
Bratzel, John F.; Rout, Leslie B. (1986). Latin American history. New York: M. Wiener Pub. ISBN091012938X.
Hole, Myra Cadwalader (1947). Bartolomé Mitre: a poet in action. New York: Hispanic Institute in the United States.
Marley, David (2008). Wars of the Americas: A Chronology of Armed Conflict in the Western Hemisphere, 1492 to the Present. Vol. 1. Santa Barbara: ABC-CLIO. ISBN978-1-59884-101-5.