Buenos Aires Province secedes after the Revolution of 11 September 1852

Background. Following the 1852 Battle of Caseros, which deposed Juan Manuel de Rosas, Justo José de Urquiza gained head-of-state power via the Palermo Protocols. The San Nicolás Agreement called for a Constituent Congress, but Buenos Aires' ruling class resisted, fearing loss of traditional preeminence. The Unitarian factions, many recently returned from exile, sought to maintain hegemony, leading to a refusal to ratify the agreement and subsequent secession.

Secession and Governance. Governor Pinto officially separated Buenos Aires from the Confederation on 11 September 1852, creating the State of Buenos Aires. The new entity asserted sovereignty, including independent diplomatic relations, and adopted a constitution in 1854. Although never explicitly recognized by the Confederation, the State maintained independence under its own government, with embassies remaining in Buenos Aires rather than the Confederation capital, Paraná.

Conflict and Resolution. The secession created a significant threat to Urquiza, prompting Alsina to order an invasion of Santa Fe. Urquiza responded with a naval blockade targeting Buenos Aires' port revenue. The separation lasted ten years, ending when Buenos Aires rejoined the Argentine Confederation following its victory at the Battle of Pavón in 1861.