Smallville Season VII Episode XVIII

Clark is sent to a simulated world by Jor-El, who demonstrates what the world would become if he were killed by Brainiac.In it, Lex is the President of United States,  starting a nuclear war with the help of Brainiac. Clark finally accepts his destiny, saving himself from Brainiac, who is then defeated by Kara. They come back to earth. Now that Brainiac is gone, Lana's fate is unknown.