His work of national organization was, however, hindered by the opposition of Buenos Aires prov, which seceded from the confederation. Open war broke out in 1859. Urquiza defeated at Cepeda the provincial army led by Bartolomé Mitre (Oct., 1859), and Buenos Aires agreed to reenter the confederation. Constitutional amendments proposed by Buenos Aires were adopted in 1860.
A leader of the revolt of Buenos Aires against Urquiza’s federal system, Mitre held important posts in the provincial government after Buenos Aires seceded from the confederation. He was defeated by Urquiza in the civil war of 1859, and Buenos Aires reentered the confederation.