When Colonial Diplomacy was first published, it contained a flawed design of the Islands surrounding Japan
Although not obvious to Newbies, seasoned veterans playing Japan quickly realised that if they were attacked, a timely retreat to Sakhalin meant that it was then impossible to destroy that unit. All the Japanese player then had to do was hang around and be included in any draw.
To offset this oversight, Avalon Hill published Errata that included a land bridge from Otaru to Sakhalin so that it would then be possible for a foreign power to attack Sakhalin with two units and prevent the Japanese advantage.
Sadly though, this only expands the problem as now the shrewd Japanese Emperor when under attack, ensure that he retreats a unit to Sakhalin and another to Otaru. These then give mutual support and the foreign power is again left helpless while the Emperor awaits a draw.
Some GM’s try to offset this by not allowing Japan a place in the draw if reduced to just these 2 units. However, this means that other players must use 2 units to ensure Japan stays in this position and also lose 2 potential japanese sc’s that might be needed to stop an opponent in another part of the board.
More importantly, by Japan having this advantage, it does not make tactical sense for Russia and China to attack Japan in the early stages.They can only gain 3-4 scs that will need to be divided between them and then they have to keep 2 units in the area to keep the Japanese fleets couped up for the rest of the game.Not really a good incentive to take on the land of the rising sun too early!!
The Moulmein convention is an adjustment to the board to include a further land bridge between Akita and Otaru. This removes the possibility of Japan forming an unbreakable enclave in the north. It also removes the rather bizarre concept that Japan would not have free movement between his home Supply centres (especially when he IS able to move freely between Kyushu and Kyoto).
There is also an argument and historical precedent for a land bridge between Sakhalin and Vladivostok,. However, this would then make it possible for other nations to actually stop Japan getting ANY builds in the first Adjustment phase no matter what opening Japan made. This would effectively make Japan a very weak power and clearly is against all the principles that the game designer had in mind.
The reason for naming this adjustment rather than just putting it amongst the variations, is to try and get it universally accepted as the norm (until Hasbro can readjust their errata). Colonial diplomacy has suffered in the past because of the failure of play testing to point out these flaws and leaving players feeling a bit cheated once they come to light. By stipulating at the start of a game that it is being played under the Moulmein Convention means that if it does become accepted, the variant as a whole can be based on a firmer footing and possibly increase it ‘s enthusiasts. At the same time it encourages those players when they GM to also use the same adjustment themselves.