Well 15 million dead Bajorans is not a small number.
It's still just a number. An angry Bajoran number at that, from "The Darkness and the Light". What does it mean? That without the occupation, nobody would have died? Mortality in humanoid species tends to be 100% in all circumstances. The rest is interpretation.
There was also rape and sex slavery.
There's that in the United States today. Whether in smaller or greater amount, it's hard to tell. Whether the US government is more or less involved, again hard to tell. The "problem" with televised Trek is that everything seen is peanuts: there are extremely few people involved in X because actors are expensive. We see about four officers abuse their powers while assigned abroad, which is
definitely below the US average (or Dutch, or Malaysian, insert-a-nation, insert-a-corporation).
Plus it did seem like the Central Command's plan was to work the Bajoran population to death to make room for Cardassians to colonialize the planet. It was not a plan to kill as many Bajorans as they could right away, but they still seemed to have a nasty end game.
Where does this come from? Only one Cardassian ever spoke like that, and this was the
fake Gul Darhe'el who only pretended to have been cruel. The real deal might have held similar sentiments or then not; might have been under Central Command or Detapa orders or then acting alone; might have been power-mad like Nazi Gauleiters or then merely mad but without powers.
Even Gul Dukat in his moments of utter madness in "Waltz" only spoke of slaughtering Bajorans in the "what if?" or "perhaps we should have" sense.
Mind you, the writers danced with great virtuosity here. They never outright established the Cardassians as "as bad as the Nazis" or "this much worse than the Ottomans" or "almost as cruel as the Crusaders" or anything. Everything was left ambiguous, open to interpretation, open to comparison with one's preferred real world conflict. And Bajorans were never given a free pass: they were a bunch of religious fanatics, ruthless terrorists, spineless collaborators, demonic opportunists, and sometimes all four simultaneously. Projecting real world conflicts on them was made maximally easy and minimally unambiguous as well. So we won't find objective proof of Cardassian war crimes or Bajoran ruthless terror doctrines, because the elimination of the objective viewpoint is how the writers make DS9 feel so real.
Interstellar law is pretty useless if it only bans the most extreme crimes, like blowing up planets and is designed to allow governments with no respect for others to save face and get away with pretty nasty stuff.
Comparison to real world international law would seem to support this (the fact that it indeed is useless, that is).
The fact is the Cardassians got away with treating Bajor as they did because nobody cared enough to risk war with the Cardassians in order to stop them.
What "risking of war"? The UFP
was at war with the Union for several years during the latter's occupation of Bajor. And we know it was pretty much Gatling guns against sharpened avocados, as per "The Wounded": Bajor could have been liberated with little effort.
But probably not without bloodshed, as even people armed with sharpened avocados can die en masse even if they can't kill much.
Anyway, the reason great powers don't crush petty dictatorships is other great powers. Would the UFP crushing Cardassia have given the Klingons an advantage at a bad moment, perhaps? Even if only by establishing the UFP as the crushing type, thus depriving it of any moral superiority vs. the Klingon Empire.
We don't really know the UFP's attitude towards wars against petty foreign dictatorships. Significantly, there is no canon declaration of even a single year of peace for the Federation; it seems they are constantly at war with at least half a dozen opponents, and nobody but the actual frontline troops even notices, not when the enemies are as petty as Cardassia... Is this because everybody wants war with the UFP (possibly knowing that there will be no crushing involved, even though victory is out of the question), or because the UFP wants war with everybody?
Timo Saloniemi