Pharoah's Plague
The massive release by the Garronean government of anti-Aslan chemical and biological agents on ihatei occupied worlds in 1129-1130. The agents were mixed into a complex "cocktail" consisting of viral and bacterial agents and chemical radicals. Their effects ranged from severe discomfort, to death. On most of the worlds where they were released, these agents killed or maimed millions of ihatei, allowing the Garrone military to quickly reclaim these worlds without much fuss.
The backlash from this atrocity was immediate. Neighboring governments condemned and isolated the Federation. The Aslan were incensed, and the Federation was only saved from retaliation by the fortuitous arrival of Virus in the Aslan Hierate. But the greatest damage was done to the ecosystems of the liberated worlds: many of the agents interacted with the local ecologies in a destructive manner, and affected local human communities as well. The ecocide led to the demographic Earthquake that saw about 40% of the local population leave for elsewhere. Most of these worlds are still affected by the long term effects of the Plague.