Perhaps for Neutral nations… when you invade, multiply the amount of enemy alliance IPC values in adjacent territories by 2, and that many infantry defend. For example, in revised, if Germany controlled Western Europe and UK invaded Spain, it would have 12 German infantry spring up on defense (representing political pressure being applied to Spain because of its adjacency to German territory, followed by an attack from Germany’s enemy). On the flipside, Germany invading Spain would have no defensive forces pop up (clearly this means that there must be adjusted IPC values to make this rule work, but of course invading Spain even if Spain gave no resistance would be very risky as it gives Britain more ways to strike mainland europe, and you probably wont be reinforcing it with the 12 infantry that would have helped you otherwise).
2 variants of this idea:
-Instead of multiplying by 2, take the amount of enemy alliance IPC values in adjacent territories and simply add the neutral nation’s IPC value to find infantry amount.
-Instead of multiplying by 2, multiply by the value of the territory (which would obviously mean that neutrals would need to be either low in value, or not surrounded by massively valued territories).