 In the north, conflict took place with organized armies under Constitutionalist generals such as Pancho Villa and Alvaro Obregón; and in the center, particularly the state of Morelos, peasants pursued guerrilla warfare.The Constitutionalist faction won the civil war, and Carranza was elected president in 1917.The war killed a tenth of the nation's population and drove many Mexicans across the norther border into the United States.