I doubt the government in Oz was voted into power based on the promise to join the invasion of Vietnam but it may well have lost office for making that choice? I wasn't here so I don't know.
When those in power lead a country into conflict, if that action is seen to be wrong it's the moral duty of everyone who comes to that conclusion to oppose it. In the extreme, those Germans who opposed the Nazis were heroes, not traitors.
It's so much easier to go with the flow but it takes real courage to shout out in opposition, especially during the emotional turmoil of war.
