The United States supported the anti-communist government in South Vietnam. It began to send military advisors to help train and support the South Vietnamese army. The South was fighting against the Viet Cong, a communist party based in South Vietnam which was allied with North Vietnam. The Viet Cong began a campaign of assassination in 1957. In 1959, North Vietnam dramatically increased its military assistance to the Viet Cong, which then began attacking South Vietnamese military units. In the American domino theory they feared that if communism took hold in Vietnam, it would spread to other countries nearby.