While artificial intelligence long ago surpassed human capability in Chess, and more recently Go -- and let us not forget Doom -- other more complex board games still present a challenge to computer systems. Until very recently, Stratego and Diplomacy were two of those games, but now AI has become table-flipping good at the former and passably human at the latter. On the surface, you might think that it's just because these games require a certain level of long-term planning and strategy. But so do Go and Chess, just in a different way. The crucial difference is actually that Stratego and Diplomacy are games of strategy based on imperfect information.