League of Nations:
Political costs for a DOW along the Russian/Japanese border result in world outrage against the aggressor nation.
If Russia conducts a DOW against Japan, the U.S. will remain neutral 1 additional turn and will not interfere with Japanese aggression against UK and ANZAC in the Pacific since they are the “victim” nation. Fear of Communism fuels American pacifism and ties Roosevelt’s hands. The Japanese Ambassador exposes a (faked) “Caucasian” treaty between Russia, UK, and ANZAC dealing with the suppression and division of Japanese assets/territories.
If Japan conducts a DOW against Russia, the U.S. will remain neutral 1 less turn and they become a “friendly” power with the pacific allies, permitting the landing of US units on allied territories and providing sea units for a common defense until a formal DOW occurs between the US and Japan. These land and sea units upon meeting with allied forces/fleets change flags (I.E. swap US vessels/units with UK,China,USSR or ANZAC pieces) to maintain “neutrality”.