Arthur Winters

Arthur Coleman Winters was President-elect of the United States of America in the spring 2008. After "Harold Saxon" announced the existence of the Toclafane to the world, Winters served as designated UN ambassador to the Toclafane. After Winters angered Saxon (the Master), Saxon ordered Winters killed by one of the Toclafane. His death signalled 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 would seem to suggest the story takes place between November 2008 and January 2009 when, in the real world, a new president would have been elected and sworn in, respectively. However evidence in Torchwood and other stories indicates the death of Winters occurs in early May 2008, suggesting that, in the Doctor Who Universe, the American electoral system followed a different path. (Winters does not, of course, correspond to the real-life presidential candidates of 2008, nor to George W. Bush.)