He's not talking about the right of the "infringed" collective to self-defense. He's saying, I think, that when you as a 3rd party enter the conflict you are adding flame to the fire. edit: noting that I'm not trying to be pro/con a third party entering the conflict. I'm just trying to be guiltless action.

I think you're waving at a sign saying "some things are worth war" and maybe that's true but for most things that's "support for war".

In this case there's a more important point, though, that a broad understanding that war will not be allowed to change the status quo will decrease the number of wars. Building and maintaining that understanding has the expense (sometimes) of making individual wars longer, but when it is (at worst) unclear which of those effects dominates it is unreasonable to label picking one side of that trade as unequivocal "support for war".