Ex-servicemen are not all of one view about Vietnam. It certainly wasn't a defensive war. It was an "all the way with LBJ war" even though LBJ never started it.
Vietnam was an attempt to impose American control upon a small country on the basis that a Communist system was a threat to the US or US interests. Any false claim that it was to save the Vietnamese from a repressive regime was soon given the lie by the carpet bombing, napalm and chemical weapons the US used - the worst acts of terrorism ever visited upon a helpless agrarian people. Makes Darfur look like a cricket match.
Australia was then, as now in Iraq, just the obedient dog.
All praise and sympathy to soldiers that defend their homeleand from an agressor. All boos and condemnation for soldiers that lend their bodies to agression or invasion of another people's country for self-serving reasons with little regard for the inhabitants.
