A commenter demands that I define torture. This is a little squishy, but here goes:
Would I be outraged if I heard about someone doing it to an American soldier?
I don't mean, "Would I feel bad?" I feel bad when people kill American soldiers, but I don't think that, say, all Japanese soldiers in World War II were irredeemably evil. That's war: you shoot at them, and they shoot back.
On the other hand, when I hear about the Bataan death march, I'm pretty sure it's evil, and should be forbidden by a legitimate state.
Some of the things the Geneva convention describes as inhumane don't strike me as torture--I wouldn't be outraged if I learned that American prisoners had been subjected to "insults and public curiosity", though I would if that "public curiosity" included "How do they look stacked up naked in a cheerleading pyramid?"
It still leaves open the boundary question, but I think that if most of us really tried to imagine how we'd feel if, say, the Syrian government held American soldiers--or American civilians incommunicado for two years while repeatedly waterboarding them, we're probably not, most of us, that far from agreement.