The Massacre of Troy was a Covenant invasion of the colony of Troy in the Hellespont system approximately five months prior to the Battle for Earth. The events surrounding the attack on Troy are notable because it was the first known instance of ONI learning of an impending Covenant attack prior to it taking place. While ONI had cracked the Covenant communications code and determined an attack on Troy was impending, the colony was not notified due to the fear of the Covenant becoming aware of the security breach and changing their encryption methods. While ONI operatives were told to evacuate themselves and as many people as they could without drawing suspicion, the rest of the colony was caught completely by surprise and was wiped out with no resistance from any UNSC forces.
These events are referenced in the audio drama I Love Bees.