Why did the USA find fighting with Vietnam difficult?

This is for sure a very interesting question because it was the first lose in USA history. So why did it happen?

There might be several reasons why finally the USA lost the Vietnam War.

First, and in my opinion one of the most important, is the well known fact that it is always easier to defend your country because of strong motivation, than to attack a foreign nation. In this case the motivation of the Vietnamese army was much stronger than the USA soldiers mostly because of the fact that in the case of losing the enemy will control their country.

Another thing was that USA soldiers were foreigners. Hardly any of them knew anything about Vietnam and none of them were able to speak Vietnamese. They were fighting in unknown terrain and most of them were not trained for defending against guerilla warfare. The case was also about the climate which the American army wasn't prepared for. Such heat, humidity and new conditions made them fight worse.

But they didn't really care about preparation before coming because they planned to stay in Vietnam just for a while. The war lasted longer than expected, making the American people tired and homesick, which had a big influence on their level of involvement. They didn't really have strong drive, they just stayed there because of coercion. So with time they started to take drugs which were very common and easily available. They stopped caring about the war, which had crippling effects.

They also lost because of a Guerilla war that they hadn't expected. American troops were mostly supposed to fight NVA (North Vietnamese Army) but finally they had to stay and defend against guerilla warfare which was much harder because the enemy knew the terrain better, while for them it was absolutely new. The main problem in this case was based on the fact that the USA army didn't exactly know who to shoot. Vietnamese soldiers didn't vary so much from civilians so it was nearly impossible to check who was really who. Because of that, lots of innocent people died and war from the American people's point of view started to look inappropriate and cruel which soon caused the American government to be very unpopular.

In fact the USA didn't even have a lot support from allies such as South Korea, Thailand, Australia, New Zealand and the Philippines. The number of soldiers that were sent for help was ridiculously small. The number of Americans who were killed during the war was ten times bigger than the combined total of soldiers killed of all the contributing allies.

Causes of Vietnam war

1. American anti-communism - America's involvement in Vietnam started because it was afraid of communism. As soon as the war broke out between south and north they started to be afraid of Vietnam becoming a communist country because this political system would start to dominate in most Asian countries. (according to domino theory). The domino theory suggests that if one country in south-east Asia is under the control of the communists, then consequently this system will spread out to all countries around, just like in a domino game. (if one piece falls, it starts the whole series 'falling')
2. War between North and South - this conflict involved the South (Republic of Vietnam) and the North (Democratic Republic of Vietnam). The South, which seemed to be against communism, was supported by the USA and their allies South Korea, Thailand, Australia, New Zealand and the Philippines. The North was supported by communist countries such as the Soviet Union. As soon as South Vietnam started to lose America gave them big financial support which very quickly proved to be not enough.  They then gave them military advisers, but as this still didn't help they then started to send American soldiers. 
3. French return in 1945 and Vietnamese resistance - 2 September 1945 the Democratic Republic of Vietnam was created when the Viet Minh overthrew the government. Vietnam was then a French colony but France didn't even think about resigning from the Indochina. As a result they was bloody fighting for several years. In the final stage of this war France had USA support including American airman and military advisers. At the end Vietnam defeated France so they signed a peace treaty with communists from the Viet Minh. After this Vietnam was divided into two parts.