Second Demigod Civil War

Origins
Immediately after the Second Olympian War with the Titans, Gaea began waking, with plans to ressurect each of her giant children. The goddess Hera saw this as a threat, and pulled Jason Grace and Percy Jackson out of their respected camps, and placed them into the other's, wiping their memories in the proccess. Jason quickly gained the trust of Camp Half-Blood when he led a quest to save Hera from imprisonment in the Wolf House.

Nearly six months later, Percy made his way to Camp Jupiter and embarked on a quest to save Thanatos. When he returned, he was proclaimed as praetor, however, quite a number of campers opposed him, mainly Octavian. Leo Valdez of Camp Half Blood later flew the Argo II over Camp Jupiter to gather the heroes of Olympus according to the Prophecy of Seven.

Battle Over Camp Jupiter
While the crew of the Argo II dined in Camp Jupiter, Leo Valdez gave Octavian a tour of the ship floating above the forum. An eidolon was possessing Leo without his knowing about it, and caused Leo to fire on the Roman camp. The Romans were quickly stirred up into a frenzy, and the seven heroes of the prophecy barely escaped.

Battle of Charleston
When the seven were in Charleston to look for a map, Roman commandos attacked and outnumbered the seven nearly three to one. Annabeth Chase ran into Reyna midway through the battle, who warned Annabeth that if the seven were to sail to the Ancient Lands, then the Romans would march on Camp Half-Blood. Annabeth did not comply to the threat, and sailed east to Europe and Rome, leaving the Romans behind.

March of Camp Half-Blood
Over the next days, the Roman troops made progress towards the Greek Camp. The last they were seen at was in Central park in New York City.