The 1820s began with infighting amongst caudillo groups. In 1826, Bernadino Rivadavia , a unitarist admirer of European ideals and a proponent of foreign investment, became the first outright president of what was called the United Provinces of South America. He proposed a new constitution, but this was predictably rejected by the provinces, who objected to the call for dissolution of their militia and the concession of land to the national government. At the same time, the war with Brazil over the Banda Oriental led to a blockade of the River Plate and caused financial crisis in the city. These two issues brought Rivadavia's presidency to its knees by 1827. The bitter unitarist/Federalist fighting that ensued only ceased when a caudillo from Buenos Aires, General Juan Manuel de Rosas , emerged victorious. In 1829, he became governor of Buenos Aires, with dictatorial powers over the newly titled Confederation of the Río Plate or Argentine Confederation.

Rosas, one of the most polemic figures of Argentine history, was born into an influential cattle-ranching family, and identified himself as a leader of men, respected by his gauchos for his riding skills and reckless personal bravery. He was an avowed Federalist, but his own particular brand of Federalism had more to do with opposing intellectual Unitarism, with its gravitation towards foreign, European influence, than it did in respecting provincial autonomy per se. As it turned out, his rule was more about centralizing power in his own province, Buenos Aires.

He left office at the end of his term in 1832 but returned as dictator in 1835 as the country teetered on the brink of fresh civil war after the assassination of an ally of his, the caudillo of La Rioja, Juan "Facundo" Quiroga . During the following seventeen years Rosas ruthlessly consolidated power using the army and his own brutal police force, the Mazorca . Mazorca means "the ear of wheat" and this symbol was used to promote an image of national unity just as Benito Mussolini used the Roman fasces to represent Italian unity in the twentieth century. The Mazorca used a network of spies and assassins to keep resistance in check, making sure the slogan "Long live the Federation! Death to the Unitarist savages!" was chanted in schools and meeting halls. On certain ceremonial days, they ensured that members of the public wore red, the colour of Rosas' Federalists. During this time, many opponents and intellectuals fled to Europe and Uruguay to escape the repression.

Continuing a process of colonization of the interior which had begun in colonial times, Rosas sought to improve his network of patronage through the expansion of territories available for farming in the Pampas. His Desert Campaign of 1833 against the indigenous peoples was the precursor to Roca's genocidal Conquest of the Desert of the late 1870s. The vast landholdings that Rosas dealt out to "conquerors" ensured he retained a body of powerful allies. Rosas also tried to ingratiate himself with the Church, thereby assuring himself of the support of those provincial elites that had been appalled by the anti-clerical tone of Rivadavia's administration. The Church responded positively, with portraits of the dictator adorning the walls of its buildings.

Rosas managed to alienate many of the interior provinces by not permitting free trade along the Paraná, by increasing taxes on provincial trade, and by allowing the import of cheap foreign produce such as French wine into Buenos Aires which undercut provincial specialities. Though he quelled several uprisings, Rosas' bloody regime was brought to an end in 1852, in the battle of Caseros. Defeat came at the hands of a former ally, the powerful caudillo Governor of Entre Ríos, Justo José de Urquiza , since the huge cattle ranches of this state were deprived of their principal markets. Urquiza was backed by dissidents in Montevideo and a coalition of interests that desired free trade on the Paraná, including the Brazilians, British and French. After defeat, Rosas left for England to become a farmer in Southampton, where he died in 1877.