Released as the first single from their 1984 album Powerslave, Iron Maiden deliver another riff-heavy song. The subject of this rocker is the Doomsday Clock, an international symbol of how close the world is to destruction.

http://thebulletin.org/overview

On the Live After Death DVD, singer Bruce Dickinson said

It’s about the romance of war, and how we’re all repelled and fascinated by it at the same time.