Rule 508.1d

The active player checks each creature he or she controls to see whether it’s affected by any requirements (effects that say a creature must attack, or that it must attack if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of attackers is illegal. If a creature can’t attack unless a player pays a cost, that player is not required to pay that cost, even if attacking with that creature would increase the number of requirements being obeyed. Example: A player controls two creatures: one that “attacks if able” and one with no abilities. An effect states “No more than one creature can attack each turn.” The only legal attack is for just the creature that “attacks if able” to attack. It’s illegal to attack with the other creature, attack with both, or attack with neither.

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