A word of advice to anyone playing Diplomacy using the rules published in the 4th Avalon Hill edition (2000): cordelia's example listed above will not work. It also is unlikely to work with the rules from the 2nd (1982) or 3rd (1996) Avalon Hill editions; more on this ambiguity below. First, however, a brief history of some other types of unwanted convoys.
(Note: This node uses standard Diplomacy notation for its examples. While an attempt has been made to explain the various moves, readers may wish to familiarize themselves with the notation before continuing.)
Basically, an unwanted convoy is any unasked for attempt by a foreign power to convoy another power's army to a destination. The original unwanted convoy made use of an obscure and now obsolete clause in section XII (The Convoy Order) of the rules published prior to 1982, that states the following:
"4. AMBIGUOUS CONVOY ROUTES. If the orders as written permit more than one route by which the convoyed army could proceed from its source to its destination, the order is not on void on account of this ambiguity; but it any of the possible routes are destroyed by dislodgment of a fleet, the army may not move."1
Since early versions of the rules did not permit for convoy paths to be specified explicitly (i.e. England: A London -> English Channel -> Belgium.), disrupting any possible convoy path was considered to have prevented the army from moving. Of course, sneaky players spotted the problem with these rules, and started doing things like the following:
F North Sea convoys A London -> Belgium.
A London -> Belgium.
F Wales -> English Channel.
F Irish Sea supports F Wales -> English Channel.
F English Channel convoys A London -> Belgium.
In the above example, England is attempting to convoy an army from London to Belgium, using the fleet that is located in the North Sea. At the same time, the English fleets in Wales and the Irish Sea are attacking the French fleet that is sitting in the English Channel, as control of the Channel by another power is generally considered by English players to be a very threatening position. Unbeknownst to the English player, the French fleet in the Channel has also been ordered to convoy the army from London across the Channel to Belgium.
Regardless of the rules being used, the underlined French order fails, as the French fleet is dislodged by the combined might of the English fleets in Wales and the Irish Sea. However, in pre-1982 rules, the English move from London to Belgium also fails, as is indicated above, as one of the two possible convoy routes was disrupted. Another much-discussed variation of this was as follows:
F English Channel convoys A Picardy -> Belgium.
A Picardy -> Belgium.
F Irish Sea -> English Channel.
F North Sea supports F Irish Sea -> English Channel.
In the above example, the French army in Picardy is attempting to move into Belgiuma move that, for the sake of this example, is unopposed. Meanwhile, the English player orders his fleet in the English Channel to convoy the army from Picardy to Belgium; since Picardy and Belgium are adjacent on the Diplomacy map, this move is not necessary and, by doing so, the English player creates two possible routes, one by convoy and one overland, that the French player could conceivably take. While all this is going on, the Russian player orders his fleet in the Irish Sea to attack the English fleet in the Channel, and also orders the fleet in the North Sea to support this attack. Since the English fleet has no support, it is dislodged, and the convoy attempt fails.
(How the English player allowed Russian control of both the Irish and North Seas, either one of which is considered a threatening position, and why he's more interested in messing with the French army, which is not an immediate threat, are both very good questions. Thankfully these are just hypothetical situations, so I don't have to worry about providing answers.)
Under certain interpretations of the rules, since the convoy was a possible route for the French army to have taken, the otherwise unopposed move into Belgium fails. However, The Gamer's Guide to Diplomacy, 2nd Edition, written by Rod Walker and published in 1979, clarified this rule by saying that rule VII.1, which stated that a unit may move into an adjacent province if unopposed, takes precedence over the convoy rule. In spite of this, the situation was still widely discussed, with Diplomacy World #28's article on tricky convoy situations refering to the problem as a common one, despite the fact that it was, in their own words, "a red herring".2
In addition, cordelia's example of two powers' armies swapping places, due to a third power's convoy, was a valid move in these editions. However, it was not uncommon for house rules to be enacted to prevent kidnapping, whereby an army is convoyed by another power's fleets without the original power's consent.
In an attempt to clarify all of these situations, Avalon Hill modified the convoy rules for their 2nd edition of the game. The previously mentioned rule XII.4 was modified to read "[...] the order is not void on account of this ambiguity; and the army is not prevented from moving due to dislodgement of fleets, unless all the routes are disrupted."3 In addition, an additional clause was added to rule XII that read as follows:
"6. BOTH A CONVOY AND AN OVERLAND ROUTE. If an army could arrive at its destination either overland or by convoy, one route must be considered and the other disregarded, depending upon the intent shown by the totality of the orders written by the player governing the army."4
There were, of course, problems with these rules, the most obvious being that the rules were now just a tad ambiguous, given their use of the word intent. At this point, whether or not the example given in cordelia's previous writeup was legal became entirely up to the game master, who had to rule on what the intent of the players was. The usual manner in which this was decided appears to have been to ignore any convoy by another power unless the move order explicitly mentioned it. This meant that the following situation would result in a stalemate:
A Holland -> Belgium.
A Belgium -> Holland.
F North Sea convoys A Holland -> Belgium.
In the above example, the English and French armies in Holland and Belgium, respectively, attack each other. Since neither move has support, and the rules specifically forbid two armies in adjacent provinces swapping positions, the moves fail. Russia attempts to convoy the English army but, since the English order did not show intent to use the convoy, the convoy attempt also fails.
However, if the English order was changed to read A Holland -> Belgium via Convoy, then the English army would take the Russian convoy, and would swap places with the French army. (It should be noted that, in the above example, if the fleet doing the convoying was English or French, then the via Convoy section of the order would be superfluous, as the "totality of the order" makes it clear that the convoy should be taken.)
The 3rd Avalon Hill edition (1996) kept this rule unchanged, but the 4th Avalon Hill edition (2000) completely rewrote the rules, and removed the ambiguity by stating the following:
"Land and Convoy Routes
In some rare cases, orders are written so that an Army could arrive at its destination either by land or by convoy. When this happens, the following qualifiers apply:
This prevents foreign powers from kidnapping an Army and convoying it against its will."5
- If at least one of the convoying Fleets belongs to the player who controls the Army, then the convoy is used. The land route is disregarded.
- If none of the convoying Fleets belongs to the player who controls the army, then the land route is used. However, the player controlling the army can use the convoy route if he/she indicated "via convoy" on the Army move order in question.
So, in summary: prior to 1982, the various versions of the unwanted convoy, with the previously mentioned exception, were valid moves, unless otherwise forbidden by house rules. From 1982 to 2000, any version of the unwanted convoy was likely to be ignored, although it could largely be dependent on the game master. In the current edition (2000 and onwards), a convoy by another power will be ignored unless the order for the army specifically states that it should use a convoy (or it is ordered to move to a province that requires a convoy, and the only convoy usable is that of another power.)
- Rules for Diplomacy, 1st Avalon Hill Edition, 1976, p. 8
- Diplomacy World #28, Autumn 1981
- Rules for Diplomacy, 2nd Avalon Hill Edition, 1982, p. 8
- Rules for Diplomacy, 2nd Avalon Hill Edition, 1982, p. 9
- The Rules of Diplomacy, 4th Avalon Hill Edition, 2000, p. 15