War Games: Very, very basically, from deep, deep memory...
A computer about to launch a global nuclear war is played at tic-tac-toe to learn it cant win. It deduces that the only way to win a nuclear war is not to start the thing in the first place. Any other strategy is a lose, or what we call mutually assured destruction.
Its obviously more complicated than that. Even though its 30 odd years old, it worth a watch.