Sycon Computer

 was in control of the time bubbles which trapped Lenk's Sontarans. It told the Fourth Doctor that it was set up to show its race the futility of war. When the Doctor told it that its systems were failing, it decided to shut them down. The Doctor told the computer that this is an act of war, and since war is futile it was futile. The computer then turned itself off because of the logic. (AUDIO: The Eternal Battle)