Defcon is also a game by Introversion Software, famed "bedroom programmers" and developers of Darwinia and Uplink, both critically acclaimed underdog games.
Defcon: Everybody Dies is a game of Global Thermonuclear War, obviously inspired by the 1983 movie WarGames (a quote from the movie, "How about a nice game of chess?, even shows up in the main menu). It can possibly best be described as an "arcade RTS", with a simple layout of 1980-ish vector graphics and easily understandable rules of gameplay. But within this simplicity lies many layers of complexity. It would probably not be a far stretch to liken some game modes in the game to the boardgame Diplomacy, widely regarded as one of the greatest strategy boardgames developed, and often called the multiplayer equivilant of chess.
The "board" of the game is our planet. Each player chooses or gets randomly assigned one of six factions roughly equivilent with the continents, Asia being split up between Russia and South Asia (the Australians have apparantly either died out already, or everyone just assumes that they'll die from nuclear winter afterwards anyway, and so Oceania is not a faction), each with 100 million inhabitants. The player must then set up his forces in this area, and prepare for the onslaught of the enemies around him. There is no building of units in the game - the forces a player starts out with are all he will have available for the entire game (with the exception of fighter planes, which regenerate at air bases at a set rate).
The game now begins. Actually, it already has, as the placement of units happen in the first two phases, DefCon 5 and DefCon 4. In DefCon 5, nothing but placement of units can be done, while in DefCon 4, radars are working. By DefCon 3, naval battle and the use of airplanes become options, and one can no longer place units. DefCon 2 has no new options, but only extend the relatively peaceful pre-DefCon 1 phase, which if in a multiplayer game without locked teams, is an excellent time for doing some clever diplomatic moves.
At DefCon 1, your nuclear missiles become available. The come in three types, LRBM, launched from silos, MRBM, launced from submarines, and SRBM, launched from bombers. Apart from their range, there is no difference. Your goal is now to either to kill as many inhabitants of the other continents as possible, to make sure as many of your own survive, or a combination of the two. By default, a player gains 2 points for each million killed, and -1 point for each of his own he loses. The Survival mode, where only your survivng inhabitants count, is arguably a better and deeper mode of scoring, allowing for many more and more interesting strategies.
And that's basically it. The game continues on in DefCon 1 until a pre-set percentage of the total nukes have been used, at which point the Victory Timer sets in. When the Victory Timer runs out, the player with the highest score is declared the winner.
What makes the game interesting and fun, at least to me, is how the simple functions of the different units, and the psychology of the multiplayer games combine to make a surprisingly deep game. No unit has any very special abilities, unlike many other RTSs, except for nukes, which unsurprisingly make huge explosions that can level cities and military complexes, just different ranges, abilities to target certain kinds of units (battleships can't attack submerged subs, for example) and probabilities of destroying other units. It all adds up to, in one way, an advanced form of rock, scissor, paper. Where you have all three, and you might have to use both your scissors and your papers to beat your enemys rocks. Add in multiplayer psychology, the ability to ally and possibly support or even backstab your ally, and you have for a very interesting game, where strategy is more important than your numerical forces, and many roads may lead to victory.
Though in all but the Survival scoring modes, it is generally regarded that a first strike against multiple opponents can be considered among the absolutely best strategies. If you can grab so many points that any single other player would have a hard time getting as many later on (it is easier to grab more points early on, as some target cities are better than others), you have a great advantage. And probably also the nukes of all the other players inbound. | 
