The official rules:
Rulebook
Quick Start Guide:
Quick Guide
Sample Game and Tutorial
http://www.sff.net/people/jbailey/diplo.htp
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Quick Rule Summary:
DIPLOMACY is basically a relatively simple game in principle, but complex in execution. Each piece has the power of 1 and can move only one space each move. There is no element of chance. The play of the game combines careful planning as in Chess, with skill in negotiating deals and persuading other players to go with your plan.
DIPLOMACY may be played by any number from 4 to 7 players. The game is best with 7 players but can be played well with less [[not according to most people that have played it, less than 7 is usually considered a distant second best *g*]]. Although the ultimate objective of the game is for one power to gain control of the majority of the board, the game can be played well by deciding in advance exactly how many moves will be played in the game.
The moves in the game are classed as two moves a year beginnning in the year 1901 with Spring and Fall moves. A game involving 8 to 10 moves, or 4 to 5 years, makes a very good game and will take as much as 3 hours for beginners. Knowing when the game will end, each player can then plan to have a maximum number of pieces on the board at that time. The winner is the player with the most pieces on the board.
The rulebook appears ominous because of its length. Most of the book is taken up with careful explanation of the rules. It is necessary for reference but need not be read and absorbed thoroughly before starting the game.
You start the game by putting the pieces (Armies and Navies) on the board as outlined in the beginning of the rulebook [[see first diagram below]], and then begin your negotiations before you make your first moves. Armies may move to any space on land and may be convoyed by the Navy across the sea. A Navy may move to any space in the sea, as well as to any space on land that is adjacent to water.
The number of pieces that a player has on the board is determined by the number of "supply centers" which he controls. The supply centers are marked by black dots on the board. Control of the supply centers is determined by which piece last occupied the supply center in the Fall move of the game. The moves of the game are figured as 2 moves each year beginning in the year 1901. There is a Spring move and a Fall move.
Occupancy of a supply center in the Spring of the year does not mean that you have control of the supply center. You must be the last force to occupy the supply center in the Fall of any year.
During the first year, it is important to plan to occupy as many of the supply centers in neutral countries as possible. It is important to make alliances with players so that you both do not go for the same supply center and waste your energies. If, in planning your moves, there are questions as to where a piece can and cannot move, or how, then consult the rulebook.
After negotiations, each player privately writes down on paper the moves for all of his Armies and Navies. These "orders" are then read off. A unique feature of Diplomacy is that every piece of every player moves simultaneously. Therefore all orders must be read out before it is clear where each piece will end the move. Only one piece can be in one space on the board at one time. If two pieces are ordered into the same space, neither one moves unless it has "support" from another piece. Thus, a move can be wasted. Negotiations before each move help avoid wasting a move in this manner.
An Army or Navy may support the move of another Army or Navy of that country or any other country in making a move, provided it is adjacent to the space into which the move is being made. Support can also be given on a defensive basis to protect a piece from being dislodged from the space it occupies.
If one piece has the support of another in making the move and the opposing piece does not, then it moves into the empty space -- or it may force an unsupported piece to retreat out of a space. To "support" a move, the supporting piece must itself be able to move into the space under attack. Opposing pieces with equal support do not move. An advantage in force of just one wins.
Adjustments in strength are made after the Fall moves have been completed. Each player writes down his desires. If he has increased the number of supply centers under his control, he writes whether he wants an Army or a Navy and if so, where. The piece must be placed in a supply center in his home country. If he has lost strength, he decides which Army or Navy is to be removed.
Alliances and deals with other players are a major part of the game. It is almost impossible to succeed in play without discussing possiblities with other players and making the best deal that you can. Once having made a verbal deal with a player, it is not necessary that you do as to agreed to verbally. When you come to write your orders, you may wish to do other than you told one or more players. The problem, of course, is that immediately, your credibility in negotiations with those players is lost. The decision as to when to "double cross" one or more of your opponents is one of the most difficult decisions to make in playing the game.