I tend to err on the side of human rights. Ultimately, though, it's one of those things where the decision is up to you (or your culture/country), and is probably not going to be universal.

Personally? I would say step in and do something about it (diplomatically, of course, if possible). Talking (tactfully, as no-name suggested) would be my most immediate course of action. However, if the situation is one where it is overt, violent, oppressive slavery - and not the more obscure, socio-economic 'slavery' that we have today - then I would have to start the 'civil conversation hourglass' at about half full. I don't give a fuck how 'legal' slavery might be in your country - no human being deserves to be bull-whipped; stoned; beaten; branded; starved; etc., simply because they wouldn't work for you. The way I see it: If you live by that philosophy, you can die by it. If we (my country) have the means to take you out of power (effectively treating you like you treat your non-compliant slaves), then you should be beaten down and taught your place. Fair is fair, right?

I will add that such a takedown should be carried out carefully. A wanton invasion of your country is not the way to go about it, because you risk creating a bigger problem than you were trying to solve (see: Iraq). I believe that true tyrants should be overthrown, but in a way that does a little as possible to lessen the standard of living of the everyday people in the region. However, if we're talking about a complete ideology change of the region (in which case, the entire population is already on the slavery bandwagon), then it's a little more complicated. Do you invade and risk turning the younger (not yet solidified in their beliefs) population against you, because of how they perceive your occupation/force? Do you remain less heavy-handed, adhere to diplomacy and vow to never invade - showing your awareness and moderation, but possibly giving the oppressor more wiggle room with which to violate more human rights? It's a hypothetical decision that I'm glad I don't actually have to make.