On February 3, 1852, a group of allied army forces composed of twenty thousand “Argentines,” four thousand Brazilians, and two thousand Uruguayans, faced off in the fields of Caseros against the army of Juan Manuel de Rosas. The defeat of Rosas on this battlefield would also seal the fate of the Rosista Confederation and, along with it, the hegemony of Buenos Aires over the provincial states. Justo José de Urquiza, the head of the Allied Army, entered triumphantly into Buenos Aires and encouraged the provinces to convene a constituent congress to ratify a long-delayed constitution that would establish a groundwork for national unity. In a missive addressed to the governments of the provinces, Domingo F. Sarmiento announced, “H. E. Governor of Entre Ríos is interested in convening a congress: first, because he would like to rely on a well-formulated and regulated authority, under the domain of a constitution.” While the idea of a federal regime had gained broad support among the provincial elites, including those of Buenos Aires, the Buenos Aires leadership resisted the idea of submitting to the authority of the Confederation under the military command of Justo José de Urquiza. After being celebrated as the liberator of Buenos Aires, Urquiza would later be accused by Sarmiento in his Campaña en el Ejército Grande (Great Army Campaign) of being the most terrifying heir to the tyrant Rosas and “less intelligent than Rosas, if such a thing was possible.” A new military conflict began that would endure a decade. Within that conflict, a constituent process unfolded, and nationality began to acquire a new legal logic: it was reconfigured as both a bond of loyalty with the state and an international legal statute.