throw my 10 cents in. In 1965-6 A study was done showing close to 100 strategic targets that, if destroyed, would have crippled N.V.s ability to conduct the War. Most of the targets wern't targeted during the Johnson Admin. Many were during the Linebacker raids that got the North to the negotiating tables. Virtually all heavy items had to be imported through Haiphong, a blockade/mining program in '66 would have major impact, but not done untill Nixon did it near the end. Perhaps had this been done the anti-war movement might not had a chance to get going.
A Lefty wrote a book thaty said he & other Leftists @ DOD were responsible from writing daily reports to McNamara(& indirectly to Johnson) about the War. The writer said they prepared the repoprts an a manner that would show statistically that the War was unwinnable, the goal was to undermine the confidence of McNamara/Johnson in the War. The micro managment by M/J was real, not to mention the silly idea of trying to "send messages of our displeasure" by manipulating the sizes of air raids as a way of "comunicating" with N.V.
Then there's the ROEs recited in "Thud Ridge". My Dad was a C-141 pilot & flew in/out of S.V. & Thailand, 105 pilots told him the same stories of outragious ROEs.

In my opinion, we "lost" the war in Vietnam because of our own people that protested the war in the United States while the American troops won in Vietnam at every missions and engagements.

i believe america was not completly in the war ,we may have had troops there but without support of the american people we couldn't function properly.

Every since major invasion by the North was repulsed up until 1972, which was when America started to Vietnamize the war. The Tet Offensive literally destroyed the Viet Cong as a fighting and destroyed the NVA that it took them 4 years to rebuild. The problem was that the higher ups in Washington didn't want to really go after the enemy. They were afraid that Russian or the Chinese would get more involved even though the sent advisors to man anti-aircraft weapons and pilots to fly "North Vietnamese" Migs.

I agree with "gman992", the Tet Offensive was a great disaster but, the mass media of the USA portrayed it as some sort of victory because the US Government had stated the Viet Cong could not launch a massive assault.

The USA could not win for a few reasons;
1) The administration provided the military with to many conflicting rules of engagement, that insured a no win situation.
2) One weakness of a democracies are wars of attrition. After five years or so the national will starts to wane!
3) The US mass media was so inaccurate (it wasn't funny) and often believed Hanoi over the words of Washington. At the time Linebacker Two raids were considered by the mass media as killing thousands of North Viet Namese in carpet bombing and that B-52s were being shot down in numbers that would quickly deplete the force. Both assumptions were false.
4) The North Vietnamese would not quit. They had fought almost continuously for hundreds of years... the Japanese, French and, the Americans were just the last of the invaders they had fought in order to unite their country under a single government. They were willing to fight until their opponents quit.
5) After the Korean Conflict, America wanted no more ground wars against the Chinese Army. That is the reason for the hesitation about provoking the involvement of the Chinese Army. If the Viet Cong and then the North Viet Namese Army gave America all it could handle, what would the US ground forces do if China sent a few dozen divisions south to assist the North Viet Namese?

NOTE:
Ho Chi Minh asked the US Government after WW-2 for assistance in getting their independence from the French. He figured we loved freedom, we would help him get freedom for his people. America did not want to displease the French, so we said no.
After WW-2, Mao Tse-tung wanted America to assist him in becoming an independent country. The USA refused to help China, so the Soviets gladly helped. The US State Department supported Mao Tse-tung but, the CIA supported Chiang Kai-Shek! ANOTHER BLUNDER!

I don't think you ever win a war where you are not welcome. If you invaded my country for whatever reason you would only win when we were all dead.