The "War Doctor" was the warrior-like incarnation of the Doctor who undertook a radically different and gruelling path compared to his other lives. Rather than act as a healer, he accepted the life of a warrior for the sake of bringing an end to the Last Great Time War.

Originally young and robust, this incarnation set upon the warrior path for which he had been intended, and disavowed the name of "the Doctor". The Time Lords and Daleks continued to refer to him by that name, however. After becoming old and tired of battle and faced with the end of reality, this incarnation considered activating the Moment, to end the Time War by committing genocide against both the Daleks and his own people. However, the Moment's sentient interface led him down a different path by letting him view his own future first-hand. By teaming up with his later selves, his mind was opened to a heroic way to end the Time War in lieu of the deadly alternative he was going to enact. Knowing he could unite with former and future Doctors in force, this incarnation helped to save Gallifrey from destruction, although he was not able to retain these memories. Because of this, his later incarnations disowned him, believing him to have destroyed Gallifrey. The Eleventh Doctor would retain the memories, and eventually remembered him as the man who saved Gallifrey, restoring the War Doctor as an embraced and respected incarnation.

After he had saved Gallifrey, retired from his warrior role, and reclaimed his name, the Doctor's outworn body renewed itself into a younger form. He remarked before regeneration that he was "wearing a bit thin," similar to the regeneration of the First Doctor.

_{Biography from the TARDIS Data Core article, licensed under CC-BY-SA}