Why did the USA lose the Vietnam War?

The Americans did not lose purely for military reasons. There were other factors as well.
Write an explanation AND cite a source which shows the importance of the following six factors:

http://www.freerepublic.com/focus/news/2049639/posts

1. US military tactics in Vietnam
The US military tactics in Vietnam were just that they should search and destroy and drop bombs on them.

2. The unpopularity of the South Vietnamese regime
With Diem in power and the US pumping money to him the US became very unpopular because he was a horrible leader and was killing all people he didn't like such as Communists which the US wanted but he was also fine with the Buddhists burning themselves alive.

3. The experience of the Viet Cong and the inexperience of the American soldiers
The Viet Cong had much more experience than the American soldiers which helped because they knew good tactics such as Guerrilla Warfare where the American soldiers just knew to kill whoever was suspected and lived in constant fear of the Viet Cong.

4. Domestic opposition to the war in the U.S.
The propaganda in the U.S. and having the American people against the war made it harder for the government because they began to have fewer and fewer reasons to be in the war.

5. Chinese and Soviet support for the Viet Cong
Since the Viet Cong had support from the Chinese and the Soviets they had trails from North Vietnam that was giving them supplies from the Chinese and the Soviets.

6. 'But did they really lose?' Summarize the argument put forward in Source 57, and your view on it.
I think that they really did lose because even though they did have a major military defeat of the VC and NVA, Vietnam still became Communists and it did spread to other countries just as thought it would in the domino theory.