Numb 31:16; Behold, these caused the sons of Israel, through the counsel of Balaam, to trespass against the Lord in the matter of Peor, so the plague was among the congregation of the Lord.
So what happened at Peor?
Numb 25:2; For they invited the people to the sacrifices of their gods, and the people ate and bowed down to their gods.
Numb 25:3; So Israel joined themselves to Baal of Peor, and the Lord was angry against Israel.