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Rules are needed for clarity, and ethical and noble issues should be resolved through mutual agreement.
Why isn't a draw considered? Simply because the attacker can often initiate such stalemate situations, especially when the opponent has a lot of slow units. I'm not even mentioning the fact that such a situation can arise in a battle of weak units. Otherwise, it's simple: if you attack and can't win, retreat.
Why don't troops fight? Because it's pointless to force the player to make deliberately losing moves.