If those armies are being convoyed to DIFFERENT locations, then no...both convoys go through as long as no other factors are involved.

If those armies are being convoyed to EACH OTHER'S location, then yes...but both are disrupted and cancel each other out as long as no other factors are involved.

The only way to disrupt a convoy is by dislodging the convoying fleet.

Thank you very much

Wait, so if army A is being convoyed to the location of army B, but army B is being convoyed to somewhere else, there is no disruption?

Darwyn's answer in the 2nd paragraph is either wrong or I misunderstand it.

A & B are land with armies. X & Y are sea with fleets doing convoys.

A->B and B->A fails as units bump
X->Y and Y->X also fails
A->X->B and B->A succeeds
A->X->B and B->Y->A succeeds

Trading places without a convoy is not possible. Trading with one or two convoy is legal.

A round robin is also legal. A->B, B->C, C->A succeeds.

TDM6, back to the original question.

For A->X->B, B->Y->C, if the second succeeds the first does also. If the second fails the first is resolved as an attack on yourself and the unsupported attack fails. The answer to your 9:19 post is no disruption.

Since you are getting close to this one, here is the ugly case to watch for

A->B, C supports A->B C->D. If the last succeeds, so does the first. If the last fails, the first is resolved as an attack at 2 on 1. The attack of A succeeds. C is dislodged and must retreat.

Darwyn's last sentence is correct.

A convoy is only disrupted if the fleet is dislodged.
Attacking the fleet has no effect unless it is dislodged.
If the destination has a unit in it, enemy or friendly, the action is resolved as an attack.

I was following all of that until "A->B, C supports A->B C->D."
How is C both supporting A->B and moving to D?

The last one is the seldom used "I'm an idiot move". Check my games and you'll find several variants. I'm good with strategy, bad with typing.

Make that A->C C supports A->B B->D.

Just to make something clear, army A is not on the same "team' as army B.

That doesn't matter as long as the orders are as stated.

Yes, I should be clearer. It is not possible for you to dislodge yourself or to support dislodging yourself. The attack exists for the purpose of blocking someone else, but not for attacking yourself.

Yes, you might be clearer....

"A->C C supports A->B B->D"

Huh?
C supports A->B does nothing, because B is ordered to D, not C
A to C fails, because it is an unsupported attack on an occupied C
B to D succeeds or fails depending on what D is doing.

Sorry, I meant C supports A->B does nothing, because A is ordered to C, not B. Jeez, you got me confusing myself now....

And having successfully both made a fool of myself and scored an assist I am going away now...

Its good to keep in mind that disrupting a convoy means that a fleet involved is dislodged from its space. It is not the same as two units bumping each other back.

Here is the first place that I ever heard of two units switching places. I understand that the rules of Diplomacy allow this but I don't recall it ever being explicitly stated. The idea that an army can march from Spain to Portugal while the army on Portugal sails back to Spain using the Mid-Atlantic fleet makes common sense. Somehow I feel that it is against the spirit of the rule that two units cannot exchange places.

Can anyone quote me the official rule that allows this kind of swap, or is it a matter of interpretation?

"Exchanging Place via a Convoy
Two units can exchange places if either or both are convoyed. This is the exception to the earlier rule that stated, "Units can't trade places without the use of a convoy.""

http://www.wizards.com/avalonhill/rules/diplomacy_rulebook.pdf
(It's on page 16)

Darwyn's response is generally correct but may be incorrect in some cases. For instance, it is possible to convoy one army from Kiel to Sweden (via Baltic Sea) while simultaneously convoying another army from Sweden to Kiel (via Skagerrak, North Sea, and Heligoland Bight).

Similar possibilities exist between Norway and most territories in England, between London and Belgium, between Spain and Tunis, between Tunis and Naples, between Albania and Apulia, or probably quite a number of others.