Lee Harvey Oswald

 assassinated U.S. President John F. Kennedy.

In 1938, the crew of the Teselecta referred to an unspecified incident related to someone named Kennedy, as an example of how time could be rewritten. (TV: Let's Kill Hitler)

On 22 November 1963, James Stevens appeared in Dealey Plaza in Dallas, Texas, from nearly eight years in the future, and rushed to the fifth floor of the Texas School Book Depository to find Francis Cleary overpowering Oswald and knocking the latter unconscious. In this version of history, Stevens bludgeoned Cleary and attempted to shoot the Master but the rifle's sights were damaged in the scuffle and Stevens inadvertently killed Kennedy. (PROSE: Who Killed Kennedy)

Meanwhile, the Ninth Doctor was photographed in Dealey Plaza in Dallas, Texas. President Kennedy passed within a few feet of him an instant before being fatally shot by Oswald. (TV: Rose; WEB: Defending the Earth!)

On 14 September 1971, the Third Doctor explained to James Stevens that the Master had sent Francis Cleary back in time to kill Oswald in order to prevent Kennedy's death and thus cause an historical embolism which would result in the Doctor himself being erased from existence. (PROSE: Who Killed Kennedy)

On 23 November, the front page of the Daily Standard reported the capture of Oswald. (TV: The Cambridge Spy)

On 23 November 1988, the Seventh Doctor asserted to Ace that Oswald was influenced to kill Kennedy by the Nemesis comet. (TV: Silver Nemesis)

Behind the Scenes

 * Doctor Who premiered with An Unearthly Child on 23 November 1963, the day after Lee Harvey Oswald killed President Kennedy.
 * The non-canonical Dimensions in Time charity special featured Pauline Fowler reminiscing to Kathy Beale in 1973 about where she was when Oswald killed Kennedy nearly ten years earlier, and telling her not to tell Pauline's husband.
 * In Asylum of the Daleks, two guest characters were Harvey and Oswin Oswald.