Axis and Allies Europe edition: Neutral Territories?
For years a group of us in Michigan have played a set of house rules that includes a diplomacy system where you can pay for diplomatic overtures to neutrals to try to sway them to lean toward you ( adding their economy to yours 1$ for sumbness like Rio de Oro, 4$ for Switzerland) and letting you move through their territory or base there.  If you get a spectacular result (11+ on 2 d 6) or two partial results they join you activating their armed forces in your color (rangeing from 1 inf for Rio de Oro to 3 inf a tank and a fighter for Spain).  I agree that attacking one European neutral should not sway the opinion of Asian and American neutrals.