A unit can't reatreat to any territory a bounce occured, any occupited territory, or the territory the attack came from.

Another way to phrase that is:

A unit can only retreat to territories it could have moved to unaided during the movement phase.

@alderian - that isn't strickly true. If vie is attacked from gal with support from boh and a unit moves from tyr-mun; then vie can retreat to tyr even though it couldn't have moved there unaided in the movement phase. Sorry to be miss picky-pants

Actually there IS a possibility you can retreat to X. If X goes to Y via convoy, you can retreat to X... Granted, there are two interpretations of convoys over adjacent territories... this site says you CAN retreat if someone attacks you via convoy (since it is a valid move you could make to 'switch' territories).

@Maniac: If vie-tyr happened on the movement phase, it would have succeeded (since tyr-mun). I don't see how you are saying that couldn't have moved unaided?

Erm, yes it could :S
Alderian doesn't mean if no-one else moved, he means that you should be able to substitute in any retreat order as an unsupported move order. However, thinking about your example I just wonder if there is a similar situation that would provide a counter example (using a cycle and blocker)

Now I undersatnd what Alderian meant, but I think he is still incorrect. Suppose bud-vie; with gal support; vie-boh, boh support holds mun. Tyr-mun; berlin supports tyr-mun. The attack on vie succeeds and vie can retreat to tyr even though he couldn't have moved there during the movement phase as it was only by him moving (unsuccessfully) to boh that allowed tyr to move to mun. (i hope that was clear)

Hrmm. Your point 1 was incorrect, but your point 2 is indeed valid. I suppose I could add on some statement about assuming all supports cut were still cut blah blah blah.

But I have to give fortknox the +1 for pointing out that you can retreat to where your dislodger came from, if the dislodger came from an adjacent territory, but via convoy.

For the record, Alderian, your explanation is 100% correct and encapsulates my rule in it. Several people may not know about the 'convoy swap', but if you do, you can see how the retreat would fit in your explanation :)

I only knew because of the big move discussion about that a day or two ago... went through the rules with a magnifying glass along with the test cases (there are a few 'interpretations' that can go multiple ways, my example in this thread being one of them. So it is nice to know what webdip does in those situations and why doing a convoy on adjacent provinces to be silly is a bad idea)

^^ Although it can be very useful if you want to switch a fleet and army

Wow, a relatively simple question turned into quite a complex discussion about the technical details of the rules.

Lixyl, I hope you got the answer that you were looking for. The simple (albeit slightly inaccurate) answer is generally no.
The exception is, as fortknox pointed out, when the attack came via convoy (and X is adjacent to Y), however this a highly unlikely (but not impossible nor irrational) situation.

Note that this exception is not universally recognized, but it is recognized on this website's implementation of the rules.
See http://www.webdiplomacy.net/datc.php
under "Choices", item 4.A.5

Example where attacking an adjacent territory with a convoy is not irrational:
imagine in a gunboat game, you have scenario where a tactical advantage could be gained by swapping units in two adjacent coastal territories, but one of these units is an army belonging to you and another is a fleet belonging to another power that has shown that they are willing to cooperate, but does not seem to anticipate or realize that you wish to swap those units with a convoy. One way to signal this is to dislodge their unit, but via convoy in order to give them the option to retreat into completing the swap. Hopefully the player would see the unusual attack via convoy and understand your intentions and not interpret it as hostile.

Consider the following situation.

Germany, an army in Belgium and Munich
England an army in Holland and fleet in North Sea and English Channel.
France an army in Paris.

Germany and England are friends and they want to swap the armies in
Holland and Belgium.

At the same time Germany wants to prevent the move from Paris to Burgundy,
but without leaving Munich.

How do Germany and England cooperate?

Lucas

Germany moves Belgium-Burgundy and Munich-Burgundy. England moves Holland-Belgium via convoy with support in English Channel. Germany is dislodged by England and can retreat to the open Holland. Munich hasn't left unless Paris supports Munich-Burgundy (and why would France do that?).

@lkruijsw and Katsarephat: brilliant. thanks for the excellent example of a scenario where this rule could be rationally used for tactical gain.