But still, while your argument has a point, I think it is flawed.
It could be used against you to say "If other entities (countries, in this case) do not act ethically towards us, then they have no right to expect us to act ethically towards them."
Better, I think, to say that: "We expect ethics from everyone, and the failure of one party to act ethically does not excuse opponents of that party from our requirements for ethical behavior."
In the case of the Vietnam war, I believe that it was originally couched as a stand against communism, both on ethical grounds (communism is bad), and on the "threat" the expansion of communism posed to our country and the world.
Hindsight shows that it would have collapsed anyway (probably, and that cracks the door on a whole other debate topic), but hindsight is not available to those who are being viewed through it.