Why did America lose the Vietnam War?
Why America lost the Vietnam War: I can't give a good answer to this, but here are a couple of points that are germane.
Ho and Giap made an aggressive attack on the national will of the U.S. They did this by issuing propaganda and performing actions that affected the politics of America. Their hope and intention was to destroy the U.S.'s will to continue the struggle.
Many, many of the guerillas were more than ordinary soldiers. Many of them held deep political motives: opposition to foreign rule and opposition to rule by any Vietnamese government that was allied with foreigners. The communitst among them had the additional political motivation of thirst for power for themselves.
Most American soldiers were ordinary soldiers with no sense of having a political or economic stake in the outcome of the war.
Once propaganda and the guerilla phase of the war had worn out the political will of the U.S., Ho and Giap had regular army forces to impose organization upon the country, their organization, their governance.
Some further insight into the political aspects of the war can be had from The Sling and the Stone by Thomas Hammes (2004). There are probably better books to answer your question, but I am not familiar enough with any others to recommend them.
There are many veterans who would argue the war was never lost by the US, rather it was lost two years after we had left the fight in 1975 by the South Vietnamese Army and government. I don't know that I completely agree, but I see their point.
So if I come at this from the angle of why we were not able to achieve a decisive victory, a number of factors, many of them true to virtually all insurgencies, come into play.
1) Distance - a 10,000 mile supply line was costly and inefficient
2) Motivation - NVA and VC troops were supremely motivated in their fight, while the South Vietnamese Army mostly was not. Many American soldiers, too, had a primary purpose of serving out their year in country and then returning home
3) Terrain - Hostile to invasion, and easy to defend, Vietnam's swamps and jungles were well known to the Vietcong and NVA, and known mostly by map and reconnaissance photos to Americans. They were fighting on home turf.
4) Guerrilla Tactics - the VC attacked us when we were isolated, then ran into the jungle, sniped at our soldiers, set off booby traps and generally bled our forces to death, while rarely coming out to fight on our terms.
One can point to many reasons why America lost the Vietnam War. I think that one of the most elemental was that as a nation, America had never waged a conflict in such a setting as Vietnam. The identification of the enemy, the challenges in the geographic settings, and the confusion of the overall mission and message all contributed to a lack of focus that ended up costing the Americans dearly. The premise of the mission and scope of what the actual goal was in the conflict was never entirely clear. The stated purpose of assisting the South Vietnamese was never fully understood due to corruption in the South Vietnamese government and a lack of capacity to assume leadership in the conflict. The idea of removing an illegitimate government was never fully acknowledged because many Vietnamese- South and North- came to resent American presence in the region. Finally, the idea of victory being defined by attrition never fully resonated with the American people, who ended up associating the war with more death, more cost, and more sacrifice than they were originally led to believe. In the end, all of these factors ended up contributing to a campaign that lacked a great deal of success.
There are many reasons that can be given for this and Americans tend to base their answers on their political beliefs. I would argue that the most important reason why the US lost the war is that the Vietnamese communists were too popular among the Vietnamese people. The US could not make most Vietnamese support them against the communists.
The communists under Ho Chi Minh were seen as Vietnamese patriots. They had led efforts to end French rule before WWII. During WWII they fought against the Japanese occupiers, again for Vietnamese independence. After the war, they fought the French again. Therefore, they were seen as the champions of Vietnamese independence. By contrast, the Americans looked to many Vietnamese like another colonial power that was going to try to control their country.
Because of these factors, the US lost the struggle for the "hearts and minds" of the Vietnamese. This is why the US lost the war.
As others have said, the United States went into the conflict with few clear goals. These goals didn't become clearer as the conflict went on. Essentially, nobody could really say what victory would have looked like by 1970. It was a conflict unlike any the US military had fought since the war in the Philippines at the beginning of the twentieth century, and US strategists had failed to give much thought to how to win such a conflict. I think the unpopularity of the war at home was a real factor as well. Failing to see why US troops should be drafted and sent to Vietnam in first place and appalled by swelling casualty lists, American support for the war was generally very low. By 1970, maybe even earlier, politicians calculated that they, and the country as a whole had much less to lose by withdrawing than they did by sustaining US presence.
In many ways, you could look at the Vietnam War as being similar to the current war in Afghanistan. There was confusion about the actual mission and the people on the ground were not as interested in winning "the war" as the American troops involved in it were.
Without clear objectives, almost any military solution will be impossible (if you believe there are such things as military solutions). Asking troops on the ground and in the air and everywhere else to fight a limited war without clear objectives is asking an impossible task. How could you win a war you never even declared as a war?
Personally, I think the inhospitable terrain and the guerilla tactics as highlighted in #3 were major factors that contributed to American defeat. Also, it is important to identify the pressure at home against the war and against American intervention in a series of conflicts world wide where it was hard to identity American interests.