by Avalon Hill

This classic boardgame is designed as a completely deterministic game - for a given set of moves, the results will always be the same. This, along with the limit on units (there will never be more than 34 pieces total on the board) are its two major strengths. Its weakness is in its duration - a face-to-face (FTF) game might take upwards of 6 hours - and that it requires exactly seven players. Most Diplomacy games are played by e-mail (PBEM) or mail (PBM).

There are two types of units in Diplomacy - the army and the fleet. As expected, armies can only operate on land, while fleets can operate on water or in coastal land areas.
The board represents the geopolitical structure of Europe in 1901. 34 of the land areas are marked as supply centers; centers in a player's home country are their home supply centers. Victory is achieved when a player controls 18 of these supply centers.
Turn sequence
There are five standard rounds in each year's turn. In each round, every player who can act records their moves secretly, and then all moves are revealed - and adjudicated - simultaneously. The rounds are:
Winter Builds
In this round, players who control a different number of supply centers and units adjust the number of units they control, by either disbanding to lower the number of units, or building - up to one unit per home supply center - to raise the number of units.
Spring Moves
All players give movement orders to their units, and movement is resolved.
Spring Retreats
Any players with dislodged units may retreat them (if possible) or disband them.
Fall Moves / Fall Retreats
As with Spring.
After Fall Retreats, ownership of supply centers is determined. Any occupied supply center has its ownership changed to the current occupant. Unoccupied supply centers do not change ownership.
The following orders are permitted:
A unit holding is attempting to stay in its province. It is an implicit defense of strength 1.
This instructs a unit to move to an adjacent province. Move orders to non-adjacent provinces automatically fail, unless the unit is an army AND there is an unbroken chain of undislodged fleets between the two locations giving the convoy order for that army's move. A move is an implicit attack of strength 1. A move succeeds if:
  • Its strength in moving into a province was greater than all other moves into the province, AND
  • The unit in the attacked province had less strength to hold the province, AND
  • The unit in the attacked province belongs to a different player than the attacker.
A unit which fails in its move is considered to defend with strength 1, but cannot be supported in defense. Two units cannot swap provinces (except by convoy).
A unit may support an action in a province to which it could legally move. Supporting an action adds 1 strength to the action, unless:
  • The supporting unit is dislodged, OR
  • The supporting unit is attacked from a different province than the one it is supporting an attack into, OR
  • The unit in the province being attacked is the same country as the supporter, in which case the support does not count for dislodging, but does count for determining single greatest attacker (e.g. If Germany in Province A has six adjacent armies - two from Austria, three from France, and one from Germany, an attempt to hold will fail - France's three strength attack will succeed. If, however, Germany supports Austria's attack, it will have strength three, balancing France's strength three, such that no one wins. If France only attacks with strength 2, Germany's support will not count, leaving Austria at strength 2, again leaving the German army as a beleaguered garrison).
A fleet may be ordered to convoy an army from one location to another. In this case, the convoying fleet does not move. If an unbroken chain of convoying fleets exists between an army's origin and destination, it can be convoyed. A convoying fleet is defending with strength 1, and may be supported.

Variants: To be further explained: