What was the Half Blood Civil War?

Chiron tells them that the camp has been attacked many times, and that the map inside the bunker, which details defenses for the camp, is from the Half Blood Civil War. The two sides caused so much devastation that the gods agreed to keep them separate and to use the Mist to make sure they never remembered each other or met again. One side was Camp Half Blood, and the other was a camp for Roman demigods, which is where Jason was. Jason was exchanged with Percy Jackson so that the sides would learn to like and trust each other.