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When it changes to Supremacy build up your fleet to equivalent power and when ready, verify that the AI's diplomatic stance still shows Supremacy and then send envoys to harm relations, rival the AI, and declare a humiliation war. Play normally, find the AI, send envoys to improve relations (to discourage it from declaring war on you) and monitor the AI's diplomatic stance.
The AI is not Pacifist - if the AI shifts ethics to pacifist at any time then restart. The AI is still planning to expand (when it starts to run out of room it seems to prefer the Belligerent diplomatic stance). All neighbouring empires have equivalent or weaker fleet power. Another regular empire is not in the Supremacist Diplomatic Stance or, if they are, the other empires fleet power is not overwhelming. When the AI finishes the Supremacy tradition tree it will prefer to switch to or stay in the Supremacist Diplomatic Stance if: The AI can complete the Supremacy tradition tree early, a mere 7 years in to the game, or it may complete it later such as around 40-50 years in. When the AI completes the Supremacy tradition tree it should immediately switch to the Supremacist Diplomatic Stance.
No Fallen Empires and Marauder Empires. Make sure no other empires are set to the forced spawning mode and then start a new game using an empire of your choice on a small galaxy with: Save this empire then go back to the empire selection screen and change its spawning mode to Forced. Create a new custom empire that is Fanatic Militarist and Authoritarian, has the Nationalistic Zeal and Warrior Culture civics, and which has a founder species with the Conformists, Fleeting, and Traditional traits.