There is no defensive pact. The A-C Pact is purely symbolic.
You are right if Germany guarantees Japan and Japan is attacked. But Germany doesn't guarantee Japan unless it joins the Axis.
In that case, Japan can attack Mongolia (and therefore Soviet Union), and can try to call Germany into the war.
Germany can make a few noises, some sabre rattling but then admit to a Soviet spy that they have absolutely no intention to attack the Soviets, then sign a non-aggression pact with them.
well, it makes an alliance much easier, since your oppinions of each other will be increased by 50.
I guess the problem wich guaranteeing is simply that if it meant that the members of the AC pact would guarantee each other, they would guarantee each other against everyone. Thats why the whole polish war doesnt really work historically in the game, since the soviet union invaded poland just hours after germany, which the western allies basicly ignored, which they dont in the game since that would mean they would be at war with the allies aswell. You cant guarantee against an specific threat, which makes simulating historical events a bit tricky if not impossible.
