Kronus was originally a remote colony of the Imperium of Man, later abandoned to its fate. The world was eventually taken by the expanding Tau Empire and remained a peaceful colony of theirs for some time.
Then a major conflict escalated. Necrons began to stir under the surface of Kronus. Eldar of Craftworld Ulthwé arrived to combat the threat, while Imperial Guard forces followed in retaliation of Eldar actions on Lorn V (played in the Winter Assault expansion). Tau presence was increased to defend against Imperial invasion. At the same time, Blood Ravens Space Marines arrived in search of Chapter artifacts, Word Bearers Legion of the Chaos Space Marines chose the very moment to attempt transforming Kronus into a Daemon World, and Orks led by Gorgutz 'Ead 'Unter came to start a new Waaagh!.
The result of the conflict in Dark Crusade depends on the player's actions, but the canonical conclusion as confirmed in Dawn of War II is the Blood Ravens crushing all other factions. Davian Thule gains great renown and Kronus is presumably brought under Imperial rule again.