Once you have declared an attack, you cannot leave the attack step without your opponent's permission. If you declare an "illegal" attack (e.g., you don't have the proper energy required to perform the attack), you may ask your opponent's permission to back out of the attack step. If he or she says "yes", you can go back and take any actions that you could have taken prior to the attack step. If he or she says "no", you may either select an attack that is legal or you may pass.