After winning independence from Spain, the provinces of the old Viceroyalty of the Río de la Plata fell into a long quarrel over a single question: who should govern, and from where. Unitarians wanted a strong central state run from Buenos Aires; federalists wanted the provinces to keep their autonomy. In 1828 the Unitarian Juan Lavalle overthrew and executed the federalist governor Manuel Dorrego, igniting civil war. The rancher Juan Manuel de Rosas organized the federalist resistance, allied with the Santa Fe caudillo Estanislao López, and defeated Lavalle. In 1831 the provinces of Buenos Aires, Entre Ríos, and Santa Fe signed the Federal Pact; defeated provinces joined one by one, and together they became the Argentine Confederation.

Rosas governed without the title of head of state, yet for two decades he dominated the confederation, handling its wars with Bolivia, Brazil, Uruguay, France, and Britain. The most storied of these came in 1845, when an Anglo-French fleet tried to force its way up the Paraná to trade directly with the upriver provinces, bypassing Buenos Aires entirely. At a sharp bend called Vuelta de Obligado, General Lucio Mansilla strung three massive iron chains across the river on twenty-four boats and lined the bank with cannon. The Argentines lost the battle on November 20, but the European fleet was so battered, and the campaign so costly, that the powers eventually withdrew and recognized Argentina's sovereignty over its own rivers. Argentina still marks that day as the Day of National Sovereignty.

Rosas fell in 1852, defeated at the Battle of Caseros by Justo José de Urquiza, the federalist governor of Entre Ríos. Urquiza convened a constituent assembly and gathered the provinces at San Nicolás to agree on a path forward, but Buenos Aires balked. In September 1852 the city seceded, declaring itself the independent State of Buenos Aires and keeping the lucrative customs revenue of its port for itself. The rest of the provinces carried on as the Argentine Confederation, and they needed a capital that was not Buenos Aires. They chose Paraná. From 1854 to 1861 this river city was the seat of the federal government, home to the president, the congress, and the institutions of a nation that pointedly did not include its largest city.

For most of a decade two rival Argentinas faced each other across the question of the port. The Confederation, governed from Paraná under Urquiza and later Santiago Derqui, drafted and adopted the 1853 constitution that still anchors the country. Buenos Aires wrote its own. They fought, negotiated, and fought again. Urquiza beat the porteño general Bartolomé Mitre at Cepeda in 1859 but declined to crush the city; Mitre beat Urquiza at Pavón in 1861 and did not destroy the Confederation. Out of that exhausted stalemate came reunification: Buenos Aires rejoined on the condition that it would not automatically be the capital, and the two halves merged into the modern Republic of Argentina. The Confederation had ended, but the constitution it wrote, and one of its names, endured.