Restriction on the Allies invading neutrals.
The historical example of Persia and Greece are questionable; in the first case the British and Russians had already divided Persia into “spheres of influence”, which included a military presence to protect economic interests; they could argue that they were not invading as such.
In the case of Greece, the Allies were invited in by the Greek government, albeit without the agreement of the pro-German King.
Its hard to see the Allies invading Holland, Switzerland etc.
Perhaps:
Every time the Allies invade a neutral country, they delay the entry of the USA by one turn. When America is in the war, they may not invade any neutral.