The thirty years that followed the defeat of Rosas saw the foundations being laid for the modern Argentinian state . An important economic expansion and the triumph of Unitarism ensured the conditions for the boom that followed; Buenos Aires was finally to emerge supreme from its struggles with the provinces; and territorial conquest began in earnest, resulting in the subjugation of the most important of the unconquered indigenous groups: those of the south.

Urquiza's attempt to establish a unifying constitution sympathetic to Federalist interests foundered when Buenos Aires proved unwilling to renounce its privileged trading terms or submit to his rule. The province refused to approve the 1853 constitution , which led to the creation of two separate republics: one in Buenos Aires and the other, the Argentine Confederation, centred on Entre Ríos and headed by Urquiza. This situation changed in 1861, when the governor of Buenos Aires, Bartolomé Mitre , eventually defeated Urquiza and his financially crippled republic. The 1853 constitution was, with a few significant amendments, ratified by Buenos Aires, and the basic structure of Argentine government was set. This established a bicameral federal legislature, an independent judiciary, and an executive president who would be elected for a fixed, non-renewable six-year term. In 1862, Mitre was elected to the first presidency of the newly titled Argentine Republic . Other constitutional provisions included ending trade restrictions throughout the country and promoting the colonization of the interior, a result of which was the sponsorship of the small Welsh settlement in Patagonia. Significantly, the president also held the right to dissolve provincial governments at will.

Mitre aimed for the rapid modernization of the country, focusing particularly on the capital. His achievements included promoting administrational efficiency, creating a national army, overseeing the expansion of a railway network and creating a national postal system. These initiatives were financed by foreign investment from Britain, which contributed the capital to build railroads, and greater export earnings as a result, particularly, of the important expanding trade in wool . A more integrated national infrastructure allowed a greater flow of trade and higher revenues in most of the interior. By the mid 1860s, therefore, much of the Federalist resistance had been stamped out and the term caudillo referred more to the election-riggers hired by the capital to control the interior.

The other significant event of Mitre's presidency was the War of the Triple Alliance (1865-70), a conflict that had its origins in the Federalist sympathies of Paraquay and disputes over navigation rights in the Paraná and River Plate. In it, Argentina allied with Uruguay and Brazil to defeat Mariscal Solano López of Paraguay. After this bitter campaign, Argentina secured control of the upper Paraná and the territory of Misiones.

The end of the war overlapped with the presidency of Domingo Sarmiento , the man who is most identified with the drive to "Europeanize" Argentina. Sarmiento was the arch opponent of caudillismo and was famous for pillorying the likes of Rosas, Quiroga, López and Ramírez in his literary works. Sarmiento believed that this age represented a "barbaric" era in Argentine history, its legacy holding the country back from contemporary North American and European notions of progress and civilization. These theories of progress were to impact heavily on the remaining indigenous populations of Argentina, as they sponsored those who believed in "civilizing the Indian", and helped to underpin the doctrine of the so-called "Generation of the Eighties" (the 1880s) who subscribed to imposing the nation state by force.

Sarmiento is also remembered for his highly ambitious educational policy , one element of which was the recruitment of North American teachers. He also encouraged European immigration on a grand scale.