Vladimir Lenin
Vladimir Ilyich Lenin was one of the most influential Human political figures of the 20th century on Earth. He led the Bolshevik Revolution, which transformed Russia from a monarchy into a communist state. The city of Leningrad was named in his honor.
In an alternate timeline, Lenin was assassinated by an unknown temporal assailant in 1916. The assassin was never caught and bystanders claimed that he vanished into thin air. No one took Lenin's place and, as a result, Russia never turned to communism and never grew stronger. Because of this, Adolf Hitler easily took over Russia and was able to focus on the west during World War II, ultimately taking England, Moscow and even the Eastern United States of America. (ENT: "Storm Front, Part II")
Star Trek: The Next Generation star Patrick Stewart portrayed Lenin in the 1974 BBC serial Fall of Eagles.