Comfort

 was a planet visited by the First Doctor, Dodo Chaplet and Steven Taylor.

No one on the planet could remember if the world was named "Comfort" for hope or after a pioneer, but in fact the name was derived from its status as a penal colony for Earth. A Central Computer system was set up to give each prisoner a purpose, and thus "comfort," eventually devolving into a perpetual war, and anyone who questioned the war was executed. As a result, the equatorial region of Comfort was lined with trenches that advanced or receded with the war fronts.

Steven and Dodo were conscripted into the opposite armies while the Doctor escaped and searched archives, where every action was recorded by the computer. He eventually tricked the population into thinking he was the "enemy" leader, and surrendered: because the humans were happy the war ended, they would no longer pay attention to the computer's orders.

However, after the TARDIS travellers left to avoid a war crime trial, the planet was later conquered by the Monoids. (AUDIO: The War To End All Wars)