steal opponent's resources?

What do you think about a tech or unit that would allow you to steal the opponent's harvested resources somehow? It could be unique to one civ or to all civs. I don't know what it would look like or how it would be done but I think it would make for an interesting strategy. I know you can take someone's potential resources from their "land" but this would immediately impact what they could buy. Both of these strategies (take land and harvested resources) were used in the Middle Ages so I don't see why it wouldn't fit in.