Too many killings of civilians by the American military - the My Lai Massacre comes to mind.
https://www.youtube.com/results?search_query=My+Lai+massacre+
Got to remember too that the Vietnam War was the very first war to be televised - thus the folks back home were seeing events on the front line a mere 24 hours after they had happened and like an everlasting unfolding drama, the people slowly came to the conclusion that both the war and it's cause were lost.
In fact public opinion in the USA became so anti-military that soldiers returning from Vietnam were spat upon by people waiting for them they were jeered at and made responsible for what had happened, the deaths of thousands of civilians in what cause?
As someone once pointed out at the time, the mighty American military were defeated by small men in black pajamas pushing bicycles through the jungle laden with guns, ammo and food.
Then this image of the napalm girl was flashed on to our TV screens. No words were needed. A young girl disfigured for life and for what purpose? To show the might and aggression of the US armed forces against unarmed civilians?
https://www.youtube.com/results?q=girl+burned+by+napalm+in+vietnam
From day one it was a lost cause. The Americans would not listen to either the French or the British, who told them plain and simple that you go into Vietnam and you come out mostly dead.
The British refused point blank to help out in Vietnam - another lost cause.
I pray for those who were killed in the Vietnam war - on both sides. I only hope that time will heal the wounds and that peace can come into the world.
https://www.youtube.com/watch?v=H7g8zV0_0GM