An awesome Avalon-Hill board game of strategy, tactics, negotiation, and (best of all) backstabbing.
Players play any one of 7 pre-WWI European countries and try to take over the rest. Gameplay can be best described as a cross between Risk and chess. Risk, because of the map-based, multi-player world domination aspect, chess, because the strategies are complex even though the rules may be simple. In addition, like chess, Diplomacy is the only other board game with popularity in the play-by-mail or play-by-email form.
A backstab is often abbreviated as "stab."
Player 1: What'd you do that for!? I thought we agreed to DMZ the English Channel!
Player 2: All is fair in love, war and Diplomacy.