War

The simplest card game there is — flip cards, high card wins, and go to war on a tie until someone holds all 52.

War has the rare distinction of being a card game with no decisions whatsoever. Every move is forced, so the entire outcome is determined the moment the deck is shuffled — it is a deterministic function of the initial order.

That makes it genuinely interesting to computer scientists and not to players. Whether a given deal terminates depends on how won cards are returned to the bottom of the pile, and under some conventions games can loop forever. It is often used as a first exercise in queue-based simulation for exactly this reason.