tl;dr: Researchers from CMU, MIT, NYU, and Stanford built Ataraxos, an AI that defeated top Stratego player Pim Niemeijer 15-1 (with 4 draws), cracking a game that had resisted even DeepMind's well-funded attempts. Stratego's difficulty stems from massive hidden information (decillions of possible piece setups), game lengths up to 2,000 moves, and the need to balance bluffing. Notably, training required only 16 GPUs and a few thousand dollars.