Diplomacy originaly is a board game made by Avalon Hill.
7 players fight each other with armies on a map of Europe. The greatest thing about the game is that you can not win without an alliance with one of the other gamers. The game also has no rules about treaties and breaking them making it a truely intresting game to talk about amongst the players. I created an online version to resolve the turns quickly, ontime and correctly. The rules are simple and can be found all over the internet use on one of the links below. We use the standard rules as described in:Avalon Hill rules in PDF
Special rules for this online version
We don't have a special retreat fase. You have to enter possible retreats along with your moves. This is done to speedup the game. And we have playtested it and it works fine.
You can retreat to a country only if the country is empty and there was no battle this turn in the country (bounce).
Retreats are automaticly done in this order:
  1. Given reatreat order.
  2. One of the free country's with a build which you don't own.
  3. One of the free country's with a build which you own.
  4. The country closest to you home country.
Rules Overview
Player Guide
The wikipedia pages for diplomacy
Avalon Hill
Avalon Hill rules in PDF
Diplomacy online resource
Middle Earth variant
diplomacy.snowplow.org an other attempt at online diplomacy
Cases for online gaming
tournament Rules
Game rules etc.
Wikibooks has the rules in english.
Here you can also find strategy guides, information per country, and more.

The rules in Dutch are available here:
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Other links
Dutch Diplomacy Association
Online dip the PC game
Boardgames list on boardgames.pagina.nl