As most Trek fans know, Starfleet has had a strict non-interference policy for pre-warp civilisations (now known as the prime directive) since the 22nd century. The prime directive existed for a number of reasons, but its main purpose was to ensure that alien species were technologically and intellectually advanced enough before joining the galactic community and to protect them from the influence of more advanced societies who could take advantage of them.
This rule quickly became one of the most important doctrines of Starfleet, and was probably derived from a similar Vulcan philosophy that prevented the Vulcans from contacting Earth until Zefram Cochrane made his warp flight in 2063.
However, although Humanity adopted these high standards for first contact, other species outside of the Federation were less concerned with "cultural contamination" of pre-warp societies. In fact, humanity itself has had their history manipulated by aliens behind the scenes for centuries, long before the Vulcans first introduced themselves.
In this list we're going to count down some of the most outrageous examples of alien influence on Human history that we've seen in Star Trek.
10. Quinn's Undercover Work On Earth
The Q who later became known as Quinn appeared in the Voyager episode Death Wish. Formally a respected philosopher among his people, the Continuum later exiled him and, in the 21st century, imprisoned him in a comet for trying to end his own life. He remained trapped there until Voyager finally discovered him and freed him hundreds of years later.
Once freed, Quinn requested asylum aboard Voyager, and Janeway was forced to hold a trial to decide if they should grant his request, effectively allowing him to die as he wished.
In an effort to prove the higher purpose of Quinn's life to Janeway, Q showed her three examples of Humans throughout history who were helped by him: one of history's greatest scientists, Issac Newton, a famous musician, Maury Ginsberg, and the Enterprise-D's First Officer, William Riker.
Newton was apparently inspired to write his equations of gravity after Quinn caused an apple to fall on his head, Ginsberg was able to make it to the famous and historically important Woodstock Festival because of his help, and Riker wouldn't have been born (and thus able to protect Earth from the first Borg invasion) if Quinn hadn't saved his ancestor, Thaddius, during the American Civil War.