According to Shaws book on International Law:
The Soviet Union made considerable use of legal arguments in its efforts to establish its non-liability to contribute towards the peace-keeping operations of the UN [sic]. And the Americans too, justified their activities with regards to Cuba and Vietnam by reference to international law.
Q. What were the US arguments, I assume at the UN, with regards to its war in Vietnam?
If it's not clear, I'm not asking here about the political dimensions but it's legal dimensions; (although the political dimension would be welcome).