What is the Time War, and how did it change the Doctor?

The Time War was a devastating conflict that occurred between the Time Lords and the Daleks, two of the most powerful and ancient races in the universe, in the long-running British TV show Doctor Who. The war is said to have lasted for centuries, and it involved both sides using their most destructive weapons and technologies, causing countless deaths and the destruction of entire planets.

The Doctor, who is a Time Lord himself and the protagonist of the series, was forced to take part in the war, something that went against his peaceful nature and his race's non-intervention policy. During the war, the Doctor committed a terrible act, which caused him to become guilt-ridden and traumatized, and led him to believe that he was the last surviving Time Lord.

The Time War significantly changed the Doctor's character and his relationship with his companions, as he became a darker, more brooding and cynical figure, haunted by his past and trying to make amends for his actions. The war also had long-lasting effects on the Doctor Who mythology, as it led to the destruction of Gallifrey, the home planet of the Time Lords, and their subsequent disappearance from the universe.