Kurukshetra war

The  was a time war waged by the Pandavas against the Kauravas. King Jarasandha foresaw that the Pandavas would overthrow him, their advisor Krishna would be killed, and this would bring an end to the Dvapara Yuga; in response to this, he arranged several plans to stop Krishna from dying. This plan succeeded, and Krishna survived the war, but the gods took notice of the aberration in the timeline and arranged for Krishna to be killed in another way. In this, the laying of the thread was completed, history was rewritten to suit the gods, and the Kaliyuga started. (PROSE: Dharmayuddha)

River Song noted a theory that the SporeShips were leftover weapons from the war that wiped out the previous universe. (AUDIO: Signs)