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 …showed first 75 words of 1532 total…

…showed last 75 words of 1532 total…Domino, due to the mass amount of expenditure, rising casualties, horrible situations on the battle fields, these were all broadcasted to US, but what was most important was that the aims were unclear, especially when they were collaborating with a Dictator to fight for the freedom, and inhumanity crisis such as My Lai by US troops. These caused big protests and engaged withdrawal of US from Vietnam and failing to save from falling to Communism.