Before 1853, Argentina formally was called the Confederation of Argentina. In 1852, one of the confederate states, Buenos Aires, declared independence. To prevent other states from leaving the confederation, the government in 1853 took on a constitution, making the confederation a federation. In 1860 the constitution was adjusted and the countries name became formally Argentina.
On September 17th, 1861, a major battel took place between the armies of Buenos Aires and the federation. This Battle at Pavón ended in a victory for the armies of Buenos Aires and its leader Bartolomé Mitre became the Argentine president, reuniting the country again.