The Time War was a conflict fought between various factions and alliances, including the Federation Alliance and the Borg across many eras of history within the Milky Way Galaxy. Although the war may have manifested itself in multiple forms depending on points of view, the apparent cause of the conflict from the perspective of Unity Starbase's crew was Colonel Phillip Green using an assimilated Guardian of Forever to create a Time Lock in which no "grandfather"-style paradoxes could exist. The changes made to the core timeline during the duration of the lock would not cause a butterfly effect ripple. In effect, the entire war would be contained within the lock. Colonel Green's plan was to assimilate what he could of the galaxy and then disable the lock so that paradoxes would revert and he could control everything assimilated via the Borg. (Star Trek: Unity)
