Oracle Text

At the beginning of your upkeep, sacrifice Peacekeeper unless you pay 1W.

Creatures can't attack.

Card Rulings

4/1/2008 When the ability resolves, you choose whether to sacrifice the creature or perform the other action. If you can’t perform the other action, then you must sacrifice the creature.
4/1/2008 If the creature is no longer on the battlefield when the ability resolves, you may still perform the action if you want.
4/1/2008 Peacekeeper’s ability doesn’t cause the combat phase or the declare attackers step to be skipped.