It's fun, but you have to overthink it way too much to properly play.
For those who don't know, Diplomacy is like Risk except without dice. Also everyone hates each other by the end.
Units (Armies and Fleets) are all equal strength and can can either move to an adjacent territory, stay still, or support another unit's movement. An army attempting to move into an occupied area will fail to move unless the amount of support to move is greater than the support of the defending unit. Players submit orders in secret, then all movements are calculated and carried out simultaneously.
Controlling a Supply Center (the red dots on the above map) allows the country to build additional units. Winning is achieved by controlling the majority of the supply centers on the map or, more often, by all surviving players agreeing to a draw. A turn can last days and a game can stretch over months. I've heard tell of single-night games involving lots of arguing and secret conferences in private rooms, but have never seen one in the flesh.
In order to get anything done you must cooperate with another player to get the supports you need for a proper invasion. But orders are secret. Now you see the fun part, because the only guarantee you'll ever have in a strategy is your ally's word. Players spend hours staring at the map, planning for all potential outcomes and weighing those against the touchy-feely notion of trust.
Player personalities have developed because of this. Here's a brief rundown of those I've encountered, complete with made up names!
