The Battle of Xandar was an open confrontation that pitted the combined forces of the Guardians of the Galaxy, Yondu Udonta's Ravager clan and the Nova Corps against Ronan the Accuser.
The Kree-Nova War was over after a thousand years of battle when a peace treaty was signed between the Kree and the Xandarians. The Kree were not pleased and rioted constantly.
The military leader Ronan the Accuser decided to take it upon himself to destroy Xandar. To this end, he made an alliance with Thanos to obtain for him the Orb in exchange for the warlord's resources.
Taking the Orb with a skirmish on Knowhere and discovering the Infinity Stone inside, Ronan disobeyed Thanos and put the power of the Infinity Stone in the Cosmi-Rod. More powerful than ever, Ronan took the Dark Aster to Xandar.
Aware of his plan, the Guardians of the Galaxy convinced the Ravagers to help defend the planet in exchange for keeping the Orb. Rhomann Dey, bringing a message from Star-Lord, warned Nova Prime Irani Rael that the Guardians could help.