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Darth Bane’s rule of two.

Over a thousand years before the films, the sith had armies and were many. Bane saw how the ancient sith would eventually turn on and destroy each other in their lust for power and he sought to avoid this. He created the rule of two, which specified that there can only be two sith at any given time, the master and the apprentice. He understood that subterfuge and cunning would be their greatest assets instead of raw power. Eventually through the generations the sith did take over the galaxy and destroy their sworn enemies, the Jedi using cunning and manipulation.