From Halopedia, the Halo wiki
In August 2552, the city was attacked by the Covenant. Most of his young students left the city for the nearest spaceport so there weren't many children left by the time the Covenant actually landed. After seeing what the Covenant had done to other worlds, Julian took the remaining children a bank vault in the city center, hoping to get deep enough underground to avoid the worst of it, though in reality they were not nearly deep enough.
Julian and the children were eventually discovered by a team of rogue Orbital Drop Shock Troopers, who had broken into the bank vault in order to steal the gold and platinum ingots stored there. The presence of the children however caused an argument. Gage Yevgenny wanted to abandon their theft and evacuate the children. Felicia Sanderson was in favor of calling in an extra Pelican. Sita, Dale and Orrin however wanted to abandon the civilians to their deaths. Tempers flared, and the ODSTs turned their weapons on each other, resulting in the deaths of Felicia, Dale, Orrin, and Sita.
Gage, in shock, went outside to find teammate Eric Santiago, who informed him that the UNSC were falling back. Gage knocked him out with his gun and called for more Pelicans, claiming they were under heavy fire. Julian and the children, along with the rest of the survivors of the battle, were subsequently evacuated back to Earth, having been informed that Reach had just been attacked.
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