I'm sorry, docdreadly, where did you see anything about race? Newsflash, most of the world has had slavery for most of history, but in most places' slavery, the slaves were the same race—and indeed, from the same TOWNS—as their masters. But they were virtually never permitted to own weapons (the reason is evident when you consider the chief exception, the Mamluks of medieval Egypt, took over the government there).
Actually in ancient Indo-European cultures, like Rome, arming slaves was seen as insulting to the warrior-caste, to the point where if slaves were needed for some military task (say crewing a ship), they would immediately be freed. It wasn't till the Renaissance that slaves started to be used as rowers.