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.
Aircraft movement question
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When an aircraft leaves a coastal territory or island, does it cost one point to move into the sea zone adjacent to the territory/island? for example, If a plane leaves Japan on the pacific map, does it cost one movement point to move in to sea zone 6? The way I read the rules, it does.
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Yes, it does cost a movement point.