That is basically redefining the word defense, though.

I can’t be like “it was self defense” if I beat somebody up because they are getting too big at the gym and they could beat me up later if I don’t beat them up first.

That doesn’t mean such a thing is never ever justified, in international relations, it just ain’t “defense”.

Yeah, by such definitions any country is justified to wage a war and always find a way to claim it’s for self-defense (which is indeed how most causus belli have worked throughout the history – they always claim to have the moral high ground when launching the war). This is also how essentially in every country it’s called the Department of Defense (unless you’re Trump) but that means nothing as they start wars all the same. Not a trace of any rules, accountability or restraints still remains under such a framing.

So while many of the reasons are questionable (understatement of the year), let’s focus on the last one. After America lost the war in Vietnam, what happened to those neighboring nations? Did they suffer from Vietnamese communists? The only Vietnamese intervention was in Cambodia, and hardly anyone thinks that wasn’t the right thing to do.