All governments in the world try to cover up, an event were known enemy saves a them from one of their allies. It's the mass medias job to expose such sneaky tactics. But i haven't seen any media in OP so far?
Far more suspious is W.G.'s atempts to hide the truth of how it was founded. And a nobility, which are normaly belongs to a royalistic contry. I guess that the nobels are the desendants, of thepeople how was victoryous in the war before the W.G. was created.
As for slaves most of the welthy contry's in our world have a history with slavory / war slaves or something like that.
But I still guess that the brith of W.G. involes some kind of dirty secret involving thier nobilety. And the actions in keeping people (like Nico Robin) from diging to far, will be cruel and merciless. Doesn't mean they are evil tough.