The State of Buenos Aires (Estado de Buenos Ayres) was a secessionist republic resulting from the overthrow of the Argentine Confederation government in the Province of Buenos Aires on September 11, 1852. The State of Buenos Aires was never recognized by the Confederation or by foreign nations; it remained, however, nominally independent under its own government and constitution. Buenos Aires rejoined the Argentine Confederation after the former's victory at the Battle of Pavón in 1861.

Regionalism had long marked the relationship among the numerous provinces of what today is Argentina, and the wars of independence did not result in national unity. Following a series of disorders and a short-lived Constitutional Republic led by Buenos Aires centralist Bernardino Rivadavia in 1826 and 1827, the Province of Buenos Aires would function as a semi-independent state amid an internecine civil war.

An understanding was entered into by Buenos Aires Governor Juan Manuel de Rosas and other Federalist leaders out of need and a shared enmity toward the still vigorous Unitarian Party. The latter's 1830 establishment of the Unitarian League from nine western and northern provinces would force Buenos Aires, Corrientes and Entre Ríos Provinces into the Federal Pact of 1831, and enabled the overthrow of the Unitarian League.

The granting of the sum of public power to Rosas in 1835 established a dynamic whereby leaders (caudillos) from the hinterland provinces would delegate certain powers, such as foreign debt payment or the management of international relations to the Buenos Aires leader. The Argentine Confederation thus functioned, albeit amid ongoing conflicts, until the 1852 Battle of Caseros, when Rosas was deposed and exiled.

...
Wikipedia