We lost Vietnam because we lied. In WWII the Allies asked Uncle Ho to fight the Japanese. He, and the people of Vietnam, did fight and almost won. After winning the war in the Pacific the Allies gave Indochina back to the French. France was fighting on the side of the Axis.
Same thing in Africa and Arabia. They helped us and then we turned around and screwed them over.
We Americans have too high an opinion of ourselves. Not every country wants to be America. Not every person wants to be an American. America starts a lot of wars and then pisses and moans about people being killed.