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.
Sub question
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Sorry if this has been addressed before, I looked but couldn’t find anything on it.
A German sub is convoy raiding sz 119, it is attacked by a British sub. The Brit misses, the German submerges. Is the German sub still considered to be convoy raiding or is that negated by the fact that he submerged? -
Submerging only removes a sub from combat. It has no further effect beyond that, so the sub will still raid the convoy.
By the way, submerging occurs before any dice are rolled by either side, so the German sub could submerge before the UK sub could fire. The only way that you can attack a sub is if either it wants to fight or you bring a destroyer to keep it from submerging.
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House rule: Submarines fire at 2 on defense if there is no enemy destroyer present…?