Vélez Sarsfield allied himself with General Bartolomé Mitre, a staunch advocate of Buenos Aires interests, following Rosas' overthrow, and served as counsel to Mitre in his opposition to the San Nicolás Agreement of 1852 (which Buenos Aires rejected). Following the province's secession, Vélez Sarsfield drafted the State of Buenos Aires Constitution of 1854 for Governor Pastor Obligado. The province's defeat at the Battle of Cepeda resulted in its reunification with Argentina, towards which Vélez Sarsfield helped craft a series of requisite amendments to the Argentine Constitution.