an important reason why i hinder the attacker, and give the defender a bonus is because of the underrepresented expansiveness of siberia. Russia is the largest country on the planet, and Siberia is some of the most rugged, underdeveloped hinterland on the planet. To assume you can just walk in and stomp out divison after division of entrenched russian soldiers, who know the land like the back of their hand is not fair.
Under my rule, you can still do it, go ahead, im not stoping you. But the russians there defend on a 3 because they know the attack is coming, and they are stretched across endless miles of forests and mountains taking pot shots at the japanese. While the japanese need to expend a huge ammount of millitary power to seek out said russians and eliminate them to gain control of these territories. Because of this their units attack power is reduced by 1. Infantry would not do well attacking into this wilderness alone.
My rule also prevents russia from ever attacking manchuria unless japan leaves it empty OR russia brings over some high power units
Also, my rule does not effect combat in china, japanese can fight russians in there as they please, if russia moves men in. because these are not origianly controlled territoreies of either power.
My rule has undergone slight textual modification, so here it is again:
House Rule 5 Russo-Japanese Non Aggression Pact
The first time one power attacks the other in one of its originally controlled territories and until the attacker captures a territory it has attacked; all that powers defending units (in originally controlled territory) gain a bonus of +1 to their defense value this phase. The Attackers units attack value is reduced by 1 during all combat against that power this turn in the opposing powers originally controlled territories. (Infantry have no attack value when not supported by artillery) The first power to break the pact, may never utilize benifits from the pact in a latter game turn.