- "We need breathing room"
- - Adolf Hitler, 1938
His ruthlessness was often compared with that of Alexander the Great, Genghis Khan, Julius Caesar, Napoleon Bonaparte, Maltuvis, Ferris, Krotus and Lee Kuan. (TOS: "What Are Little Girls Made Of?", "Patterns of Force", "Whom Gods Destroy")
Hitler was quoted as saying in 1938, "We need lebensraum," translated as "living space" or, effectively, "breathing room," in defense of his attacks on neighboring countries. Three and a half centuries later, in a conversation between Captain James T. Kirk and General Chang, Kirk would point out Chang unintentionally quoting Hitler when he made a similar remark regarding possible Klingon ambitions. (Star Trek VI: The Undiscovered Country)
A headline from a 1941 edition of the San Francisco Herald read "Hitler on the Move", followed with the subheading that stated "Roosevelt presses Congress for British aid." (TNG: "The Big Goodbye") According to Dixon Hill's secretary Madeline, the last time they had a new case was when "Hitler and Stalin were bosom buddies." (TNG: "Manhunt")
James T. Kirk mentioned Hitler to Roger Korby in 2266, after Korby implied Human programming leading to near-practical immortality as the major impact of transferring the Human consciousness into an android body. (TOS: "What Are Little Girls Made Of?") Fleet Captain Garth of Izar later boasted to Kirk, in 2269, that as "Master of the Universe", he would go much farther and have more success than other previous despots, such as Hitler. (TOS: "Whom Gods Destroy")
In 2368, Captain Jean-Luc Picard of the USS Enterprise-D agreed with faux historian Berlinghoff Rasmussen that saving an endangered planet could allow "the next Adolf Hitler" or Khan Noonien Singh to come into being. (TNG: "A Matter of Time")
Hitler proved much more successful in at least two alternate timelines.
In an alternate timeline, Hitler's success was based on altered events made during the Temporal Cold War, thanks to the actions of the Na'kuhl temporal agent Vosk. In this timeline, Vladimir Lenin was assassinated in 1916 and the Russian Revolution was prevented. With Russia in disarray, this timeline's Hitler was able to concentrate on the Western front, and conquered Belgium, France and Britain in quick succession. He followed his victories in Europe by invading the United States of America, conquering New York City, Washington, DC and much of the Eastern seaboard. In 1944, this Hitler made a state visit to New York and pledged to eradicate "financial profiteers" (euphemism for Jewish interests). Temporal agent Daniels sent Jonathan Archer and the starship Enterprise NX-01 to this timeline, and they were able to prevent Vosk's actions and restore the timeline. (ENT: "Zero Hour", "Storm Front", "Storm Front, Part II")
In a later alternate timeline, Hitler's success was in part thanks to Starfleet doctor Leonard McCoy, who traveled back in time to 1930 and saved a woman, Edith Keeler, from a traffic accident. Keeler would go on to lead a pacifist movement within the United States of America to convince their government to delay entry into the war. According to historical documentation of the alternate reality studied by Spock, the absence of American military support on the western front of Hitler's expansion would buy the dictator enough time to perfect both V-2 ballistic missile tests and heavy water experiments, ultimately winning the race to create Earth's first atomic weapons. With these devices and their launching mechanisms complete, Hitler was victorious. The evolution of this timeline ultimately resulted in the non-existence of the United Federation of Planets. This timeline was aborted when Spock and Kirk, following McCoy, prevented the doctor from saving her. (TOS: "The City on the Edge of Forever")