The Giant war.

While the gods and the seven were fighting Giants in ancient Greece, trying to keep Gaea from waking, Camp Jupiter was encroaching on Camp Half-Blood.

You were one of the Romans. You didn't like this, you didn't want to hurt the Greeks, but you were afraid of what Octavian might do to you if you opposed him. Just as you were helping other demigods load more ammo into a ballista—

Clarisse: "HEY!"

A girl in bronze armour shouted for your attention. As everyone looked up from the ballista, it was suddenly destroyed, smashed by two others with sledgehammers. Suddenly, all of you were surrounded by six demigods of Camp Half-Blood, the girl in bronze armour glaring at you all through her helmet, her spear sparking with electricity in her hand.