The Doomsday Clock ranges from full peace at 10 minutes to midnight (cannot go lower than that) to full nuclear war at 0 and is a measure of how close the world is to full out nuclear war. If at any time the clock reaches 0 (cannot go higher than that) the game allows for any player (USA or USSR) to manually trigger the Apocalypse through the Launch Order Authorization screen.
Increasing the Doomsday Clock
Offensive operations with Army Commands, Surface Fleets, Carrier Fleets, Bomber Commands, or Fighter Commands against regions controlled or dominated by enemy faction will immediately trigger a war in that region and increase the Doomsday Clock by 1 towards midnight or midnight if the region was dominated.
Reducing the Doomsday Clock
See Disarmament Focus.