Generally speaking powers defend together. However in G40 there are powers that start off neutral (Russia and USA) or not at war with each other (like Japan and UK/France). You could have a case at sea when you attack a sz with units from multiple powers, and can ignore the units that belong to a power that you aren’t yet at war with.
Odd Scrambling question
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Italy has a navy in zone 110 uncontested, on Germanys turn they bring in transports and ships for an amphib on UK in same sea zone as Italian fleet. Does the Italian fleet defend against the scramble from UK?
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Italy has a navy in zone 110 uncontested, on Germanys turn they bring in transports and ships for an amphib on UK in same sea zone as Italian fleet. Does the Italian fleet defend against the scramble from UK?
No. The scrambling is defense against Germany’s attack. So like all other battles, friendly units in the seazone are not included. On offense, only one power is involved. On defense, multiple powers are involved.