Battle of Gettysburg

The Battle of Gettysburg was a major battle in July 1863 during the American Civil War. It was fought near the Pennsylvania town of Gettysburg. 

The Third Doctor and Charlie Fisher mapped the Confederate lines and delivered the information to the Union Army, which helped them win the battle. (COMIC: Backtime)

The Eleventh Doctor, Amy Pond and Rory Williams were briefly trapped in a virtual reality version of the Battle of Gettysburg. (COMIC: Trapped in the Pages of History)

whoisdoctorwho.co.uk
The website whoisdoctorwho.co.uk had a list of sightings of the Doctor from which people had ostensibly been submitting to Clive, a conspiracy theorist character from TV: Rose, who had pictures of the Doctor's ninth incarnation on the website, asking if anyone had seen him.

A submission from Terrance Chua stated that while researching the American Civil War at the University of Georgia, Terrance had found a picture from the Battle of Gettysburg in 1863, where a man in a Union Army captain's uniform was standing next to a converted field hospital "that looked slightly out of place". What puzzled Terrance was that in these photographs, he saw another photograph of "apparently the same man", with a bandaged arm, but this time wearing a Confederate colonel's uniform. He said that both looked exactly like "this Doctor".