Cylon War
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The Cylon War was the conflict that ended over forty years prior to Re-Imagined Mini-Series. It involved the early Cylons revolting against their role of servants to the Colonials; while the Cylons used Raiders and basestars, the Colonials developed the first battlestars, of which Galactica was one.
Given that the Articles of Colonisation were signed fifty-two years before the Mini-Series (Colonial Day), it is possible that the Cylon War was what precipitated the unification of the Twelve Colonies.
Veterans of the War include William Adama and Dash.
See also: Thousand-Yahren War