Who decides what invasion is and isn't unlawful? Who makes such law on which it's decided in the first place?

Invasions are, by definition, not unlawful, since there is no international law (what is called international law is not actually law), there being no sovereign to enforce any such law.

it's more of a point that the same designation could very well apply to the said ruling class in the first place.

They are state actors, so no. They always make sure to exclude themselves.

It's "desegregation" that occurred at gunpoint.

Alternative view: it's government ensuring that people can attend school safely regardless of their color. If only it continued ensuring that, and you may not have cited desegregation as such a bad thing.