Any of a family of military simulations derived from a game written by Peter Langston many years ago. A number of multi-player variants of varying degrees of sophistication exist, and one single-player version implemented for both Unix and VMS; the latter is even available as MS-DOS freeware. All are notoriously addictive. Of various commercial derivatives the best known is probably "Empire Deluxe" on PCs and Amigas.
Modern empire is a real-time wargame played over the internet by up to 120 players. Typical games last from 24 hours (blitz) to a couple of months (long term). The amount of sleep you can get while playing is a function of the rate at which updates occur and the number of co-rulers of your country. Empire server software is available for Unix-like machines, and clients for Unix and other platforms. A comprehensive history of the game is available at http://www.empire.cx/infopages/History.html. The Empire resource site is at http://www.empire.cx/.