According to Xenosaga I & II, it started when the autonomous planet, Zoar, began invading the autonomous planets of the Miltian System. There was never an official declaration of war, and the entire truth of what really happened was lost due to the space/time anomaly which engulfed the region afterwards. Among the rumored causes, were that extremists among the people of the Immigrant Fleet (Ormus) who lived chiefly among the planets of the Miltian System and Zoar, had feared Miltia joining the Galaxy Federation.