Massacre of the Xandarians

"Thanos already has the Power Stone because he stole it last week when he decimated Xandar."

- Thor to the Guardians of the Galaxy

The Massacre of the Xandarians was the result of a mass purge initiated by Thanos in an attempt to obtain the Power Stone.

Background
"Fine. I'll do it myself."

- Thanos

As he planned to gather all six Infinity Stones to wipe out half of all life in the universe, the 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.

Massacre
Although the massacre was not seen, it is known that Thanos himself personally led the assault on Xandar and that the Nova Empire were defeated and destroyed by Thanos. Thanos then retrieved the Power Stone and executed half of Xandar's population as part of his goal to balance the universe.

Aftermath
The massacre of the Xandarians sparkled 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.