You may win the game diplomatically by convincing enough faction leaders to unite behind you as Supreme Leader of Planet. A 75% vote of the Planetary Council and the knowledge of Mind/Machine Interface is required to secure such election. Only the leader of one of the two factions with the highest vote totals may stand for election as Governor or Supreme Leader.
When a Supreme Leader is elected, a faction leader may choose to defy the will of the Council, and refuse to submit. In this case, the Supreme Leader must, with the help of loyal factions, conquer all defiant factions to achieve a Conquest victory. Because of the military power usually required to secure election as Supreme Leader, defying the will of the Council is generally fairly suicidal. Other factions will therefore usually only take this course if you have committed atrocities against them or grossly and repeatedly betrayed them.
If you win a Diplomatic Victory, you are awarded 1200 points minus two for every game turn elapsed. If Cooperative Victory is enabled and your Pact partner is elected Supreme Leader, you are awarded half this total.
A leader may not be declared Supreme Leader unless the alien factions have been eradicated. Once they have been eradicated, Supreme Leader victory is allowed normally. If a Progenitor faction is “freed” by a Probe and swears a servile Pact, or swears a servile Pact during the course of the game, that counts as “being eradicated” and any human faction can win as Supreme Leader—even if the faction to whom the Pact was sworn are themselves eradicated later on.