310.8. If a non-Siege battle’s defense is 0, it’s put into its owner’s graveyard. (This is a state-based action. See rule 704.)
704.5w If a non-Siege battle has defense 0, it’s put into its owner’s graveyard.
704.5x If a battle has no player in the game designated as its protector and no attacking creatures are currently attacking that battle, that battle’s controller chooses an appropriate player to be its protector based on its battle type. If the battle has no battle type, its controller must be chosen. If no player can be chosen this way, the battle is put into its owner’s graveyard. See rule 310, “Battles.”
