The United States and the Soviet Union could not agree on a plan for a proposed 1956 election meant to unify the partitioned Vietnam. Instead, the South held a separate election that was widely considered fraudulent, leading to continued internal conflict with communist factions led by the Viet Cong that intensified through the late 1950s. With supplies and support from the Soviet Union, North Vietnamese forces became directly involved in the ongoing guerrilla war by 1959 and openly invaded the South in 1964.