Why Did the USA Fail to Save South Vietnam from the Vietcong?

The Cold War derived from differences of ideologies between countries. The USSR was under communist system, which opposed Capitalist US and most of the Western Europe. Communism had continually spread through Eastern Europe, even threatening Greece, Italy and France, America (Cuba and Mexico since 1960s'), North Africa (Libya in 1970's) and even to the Far East. It began in 1945 when the USSR occupied the Buffer States after Yalta Conference. The US started worrying about countries falling to communism. People including Churchill from his Iron Curtain Speech in 1946 accentuated this. …