History of Vietnam since 1945 unstable as no government could keep power for long. There were more coups, often more than one every year. The Communist-run Viet Cong expanded their operation and scored some significant military victories during this period. In 1965, US President Lyndon Johnson sent troops to South Vietnam to secure the country and started to bomb North Vietnam, assuming that if South Vietnam fell to the Communists, other countries in the Southeast Asia would follow, in accordance with the domino theory. Other US allies, such as Australia, New Zealand, South Korea, Thailand, the Philippines, and Taiwan also sent troops to South Vietnam.
CIA activities in Vietnam Vietnam to treat refugees and to train Vietnamese medical personnel. The second pacification operation was launched late April 1955 in the southern Dinh Dinh and northern Phu Yen portion of Central Vietnam. Northern Vietnamese troops needed a way to link themselves with their allies in Southern Vietnam. The Viet Cong and Northern Vietnamese soldiers were able to supply troops and military operations through secret tunnels and the Ho Chi Minh trail. The Ho Chi Minh trail was an interlocking trail system that was created through the borders of Laos and Cambodia that reach from North Vietnam to South Vietnam. During