A clone of the game Global Thermonuclear War from the movie WarGames.

(From Wikipedia)

"Defcon is a real-time strategy game created by British game developer Introversion Software The presentation of the gameplay is reminiscent of the "big boards" which visually represented thermonuclear war in the films Dr. Strangelove, Fail-Safe, and WarGames.

The game is digitally distributed on several channels. Introversion is selling the game as a direct download, and it is also available through Steam. Introversion also sells a boxed version; this contains the game media and manual but will still require online authentication, as with the retail release of Half-Life 2."

The official Linux port for DEFCON has been released, and is currently version 1.42.