Bayle Shanks's website: notes-games-diplomacy

takes, omissions, or if you'd like, your own summary or tips):

You are a country in World War I. The object of the game is to win control of all (or almost all) of the 'supply centers' in Europe. Europe is divided into 'provinces'. Some provinces have a circle on them, meaning that they are supply centers.

You have armies and navies. All units are equally strong. A province can only hold one unit at a time. On each turn, you may issue one order to each of your armies and navies. You can tell them to STAY where they are (also called HOLD), or to MOVE someone else, or to SUPPORT another unit's actions. In order to SUPPORT another unit's MOVE action you must specify the destination of the other's MOVE.

If two units attempt to go to the same province at the same time, and no one is supporting either one, then they are evenly matched, so they both 'bounce' (retreat) back to where they came from. If one of them was just STAYing (defending), then it gets to stay.

If two units attempt to go to the same province at the same time, and one or both of them has other units whose current move is to SUPPORT them, then you add up the number of supports that each side has, and if one side has a strictly greater amount of support, they win and the other side retreats.

If you attack a unit which is SUPPORTing, you cut the support, and it's as if the SUPPORTing unit was only STAYing (except: you can't cut support by attacking from the province to which support was being given).

There are three types of provinces, land, sea, and coast. Armies can only go on land or coast. Navies can only go on sea or coast. Navies have an additional move type, CONVOY, which means it acts like a bridge over the sea between two adjacent coasts for armies.

Each turn is either a Spring turn or a Fall turn. At the end of each Fall turn, you count how many supply centers each player controls (the last player to occupy a supply center controls it). If you have more supply centers than your total number of armies+navies, you get to add an army or navy at an open supply center. If you have LESS supply centers than your total number of armies+navies, you must disband an army or navy of your choice.

If a unit has to retreat but other superior (= more support) forces moved into where it was, then it can retreat to anywhere else adjacent. If there is nowhere for it to go, it is disbanded (but if the player didn't/doesn't lose any supply centers, this will only be a temporary loss because they will get to add back a unit next Fall).

Before each turn there is time for 'diplomacy' where you can negotiate with others. These negotiations are not binding, ie it is NOT cheating to tell someone you will do one thing and then to do something different. As the rules say, 'Deciding whom to trust as situations arise is an important part of the game'. You can go into other rooms to try to get some privacy during negotiations (but surreptitious eavesdropping is also allowed).

There's an Android app called Realpolitik that appears to let you enter everyone's Diplomacy moves and tells you what happens.

here's the official rules:

https://www.wizards.com/avalonhill/rules/diplomacy.pdf

other notes:

each space can only hold one piece at a time
you can support a unit in holding even if that unit is attacking or giving support; because if the attack fails or the support the other unit is giving is cut, then the unit ends up holding anyways, so the support you are giving is for that
- as a special case: two units can mutually support each other in holding
you can chain convoys
convoys can still occur if attacked (unsuccessfully), they are not broken, unlike support
At each Fall turn, you add up how many supply centers you have, and then add or subtract units until your total number of units equals your total number of supply centres. For example, if you have 3 units on the board and 4 supply centres after a Fall turn, then you would get to place a new unit onto the board after the end of that turn. New units must go in supply centers in your home territory, so make sure you have enough of these provinces open.
all moves are simultaineous; in a turn, each player writes an order for each piece they own, and then all of these are 'resolved' at once
to support, the supporter must itself be able to move into the territory of the battle in which support is being sent (ie, for a move order, to the country that is being attacked, not to the country the attacker is coming from)
a unit cannot retreat into the province from which its attacker came, nor to a province that is empty due to a standoff
writing orders:
- support orders must not only say who is giving the support, but also who is receiving the support and for which attack or hold
- likewise, convoy orders must not only say who is convoying, but also which unit is being convoyed from where to where
i think: a unit being attacked, cannot cut it's attackers support, unless it has enough support in this attack to dislodge the unit (i think; see http://boardgamegeek.com/thread/301188/diplomacy-rules-overview-hope-helps)

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