The Massacre of the Xandarians was an immense slaughter orchestrated by Thanos to obtain the Power Stone. Thanos arrived on Xandar where he proceeded to wipe out half of the Xandarians and obtained the Stone, which had been under protection by the Nova Corps.
- "Fine. I'll do it myself."
As he planned to gather all six Infinity Stones to wipe out half of all life in the universe, the Mad Titan Thanos struck alliances which were supposed to bring him the Stones. However, both Loki, who was supposed to bring back the Tesseract but lost it along with the Mind Stone embedded in the Scepter, and Ronan the Accuser, who should have delivered the Power Stone, failed in their plans. Fed up with these failures, Thanos decided to go and find the Stones himself.
Thanos' quest for the Infinity Stones was to begin with Xandar, where the Power Stone had been stored within the Nova Corps Headquarters following Ronan's death at the hands of the Guardians of the Galaxy.
In accordance to their usual operation, Thanos and the Black Order traveled to Xandar and soon invaded the planet, effortlessly destroying the Nova Corps protecting it. Thanos retrieved the Orb, destroyed it and placed the Power Stone in the Infinity Gauntlet. In parallel, he and the Black Order massacred half of the Xandarians as part of Thanos' quest to balance the universe.
The massacre of the Xandarians sparked the beginning of the Infinity War. Having successfully retrieved the Power Stone, which enabled him to strike with even more might than before, Thanos immediately sought to acquire the other Infinity Stones. His quest led him to find the wandering Asgardians aboard the Statesman, where Thanos knew he could find the Space Stone contained in the Tesseract. As a result, Thanos and the Black Order attacked the ship in a successful attempt to reclaim the Space Stone.
Thor later relayed the news on Xandar's decimation to the Guardians of the Galaxy, much to their visible shock and horror, as they had almost died to save it and maintained a working relationship with the Nova Empire. Having been defeated by Thanos and sent back to Earth by Heimdall, Bruce Banner also informed Tony Stark and the Masters of the Mystic Arts about Thanos' deeds.