Question

why did America decided to intervene in the Vietnamese civil war

Answer 1

(I) The Vietnamese had to fight a long struggle first against France, and then against Japan and lastly, against USA. Following were some of the causes for the involvement of US in Vietnam :
(i) Spread of Communism : First and the foremost was the fear of the spread of Communism in Vietnam. The United States (US) regarded the spread of Communism a great danger to the capitalist countries. The US was ready to fight against Communism in any part of the world. So when Communism showed its head in Vietnam, it at once came forward to check it.
(ii) To Support France : The second cause of US entry in Vietnam was the humiliation that France had to face in Vietnam. Being the champion of the capitalist countries, US wanted to crush Vietnam at all costs to save the prestige of the capitalist countries, especially France which was an ally of the US in the Second World War.
(iii) Against Unification : USA was against the unification of Vietnam. The Geneva Conference had divided Vietnam into two parts : North Vietnam and South Vietnam. When the Ho Chi Minh government in North Vietnam and National Liberation Front in South Vietnam tried to unify the two parts, in violation of the Geneva Conference, US could not tolerate this. Hence, it decided to intervene in Vietnam.
Within no time, the United States sent a large number of US soldiers with the latest war equipment. This dragged on for many years (1965-1972).
Effects of US Involvement on Life within US itself : The US involvement in Vietnam had a far reaching effect on US itself. America had to suffer a huge loss of men and money though she caused great destruction in North Vietnam through bombardment.

Answer 2

The Vietnamese had to fight a long struggle first against France, and then against Japan and lastly, against USA.  First and the foremost was the fear of the spread of Communism in Vietnam. The United States (US) regarded the spread of Communism a great danger to the capitalist countries. The US was ready to fight against Communism in any part of the world. So when Communism showed its head in Vietnam, it at once came forward to check it. The second cause of US entry in Vietnam was the humiliation that France had to face in Vietnam. Being the champion of the capitalist countries, US wanted to crush Vietnam at all costs to save the prestige of the capitalist countries, especially France which was an ally of the US in the Second World War.(iii) Against Unification : USA was against the unification of Vietnam. The Geneva Conference had divided Vietnam into two parts : North Vietnam and South Vietnam. When the Ho Chi Minh government in North Vietnam and National Liberation Front in South Vietnam tried to unify the two parts, in violation of the Geneva Conference, US could not tolerate this. Hence, it decided to intervene in Vietnam.Within no time, the United States sent a large number of US soldiers with the latest war equipment. This dragged on for many years (1965-1972).Effects of US Involvement on Life within US itself : The US involvement in Vietnam had a far reaching effect on US itself. America had to suffer a huge loss of men and money though she caused great destruction in North Vietnam through bombardment.