Adolf Hitler (April 20, 1889 - April 30, 1945) was a 20th century human dictator who was born in Austria. He led the Nazi party in Germany during the 1920s and 1930s, and later seized total control of the German nation.
Hitler was able to tap into the anger the German people felt over the reparations forced on their nation following World War I. The power-mad Hitler used the Nazi party as a tool for governmental domination. Hitler's ruthlessness would be compared to others, such as Alexander the Great, Genghis Khan, Julius Caesar, Napoleon Bonaparte, Maltuvis, Ferris, Krotus, Lee Kuan, Khan Noonien Singh and the Vulcans of the Last-of-all-Cities colony. He also initiated the attempted systematic genocide of Humans of Jewish descent -- the Holocaust, which led to the deaths of six million Jews alongside approximately six million other enemies of the Reich. (TOS episodes: What Are Little Girls Made Of?, Patterns of Force, Whom Gods Destroy; EV comic: Cloak and Dagger, historical accounts.)
In the late 1930s, the Hitler and the Nazis turned their aggression outward when they began the conquest of Europe. Hitler tried to justify this in 1938 by saying, "We need breathing room." In 2293 the Klingon General Chang would repeat this line. Kirk replied by stating, "Earth: Hitler, 1938." (TOS movie: Star Trek VI: The Undiscovered Country)
In the face of Hitler's aggression other nations, including the Soviet Union, Great Britain, Canada, and later the United States formed an alliance to stop Hitler in his drive for conquest. The Allies drove the Nazis back to Germany and defeated them by 1945, but at a cost of millions of lives and large scale devastation of much of Europe and other Nazi occupied lands. Hitler committed suicide as Allied troops entered the city. (TOS episode: "Patterns of Force")
For centuries to follow, it was debated whether the removal of Hitler from history would have prevented World War II. Leonard McCoy dismissed this idea, and the "one-man-as-catalyst" theory of history, arguing that war was part of the human condition until the Human race outgrew it. (TOS novel: Strangers from the Sky)
In two different alternate timelines, Hitler was much more successful.
In one altered timeline, Hitler's government would receive assistance from the Na'kuhl - one of the factions fighting in the Temporal Cold War. The Na'kuhl agent Vosk was able to alter history starting in 1916, when future Russian leader Vladimir Lenin was assassinated, which in turn prevented the Russian Revolution from taking place. As a result, by the late 1930s Russia was in disarray and Hitler was able to concentrate on western Europe, conquering not only France, but Great Britain as well. Hitler was then able to turn his attention to the United States, conquering the eastern United States - including Washington and New York City. With the assistance of temporal agent Daniels, the crew of the Enterprise (NX-01) was able to defeat Vosk and destroy the Na'kuhl temporal conduit, thus restoring the timeline. (ENT episode Zero Hour, Storm Front Parts I and II)
Another alternate timeline was created when suffering from an accidental cordrazine overdose, Dr. McCoy entered the Guardian of Forever, and wound up in 1930s Earth. While there, he saved a woman named Edith Keeler from a traffic accident. Keeler would become a renowned pacifist whose efforts delayed the entry of the United States into World War II. Without U.S. involvement, Hitler's government had time to complete the heavy water experiments and missile tests, winning the race to develop nuclear weapons. With such advantages, Hitler was able to win the war, and as a result the United Federation of Planets was never formed. Fortunately, Captain James T. Kirk and Mr. Spock had followed McCoy back in time, and were able to prevent McCoy from saving Keeler's life. When Keeler died, the timeline was restored to normal. (TOS episode: "The City on the Edge of Forever")