Arthur Winters

Arthur Coleman Winters was American president until 2008. After "Harold Saxon" announced the existnece of the Toclafane to the world, Winters served as designated UN ambassador to the Toclafane. After Winters had angered Saxon (the Master), he ordered Winters killed by one of the Toclafane. His death signaled the start of the Toclafane occupation of Earth (DW: The Sound of Drums).

The Paradox Machine only rolled back time to a few moments after Winters' death, meaning that it still happened (DW: Last of the Time Lords)

Behind the Scenes
Dialogue in The Sound of Drums refers to Winters as "President Elect" of the United States, rather than the actual President. This almost certainly contradicts the intent of the story itself and Winters' role within it.