The US did not green light Vietnam. The US was forced to go over there and save the entirety of the post-WWII French army. At the time, communist movements were rising all over in the chaos of a post-war Europe and Stalin was playing his games in Eastern Europe. The fear was that if the French army collapsed in Vietnam, then France could fall to political chaos. France, as France is always wont to do, sticks to its traditions even when those traditions are bad for France and so when Germany and Japan fell, France was back at rebuilding their colonial empire-zero moral ambiguity here on the US part...so, BS to you ))