The Germans have no destroyer present, therefore the defending air units can not hit the sub, and of course the sub can never hit planes. Think of it in terms of the combined arms rule, air units can only hit subs if they are combined with at least 1 participating destroyer, the difference with this particular combined arms is that it works while attacking and defending. In the scenario that you described, the sub would destroy the remaining transport without rolling and the defending fighters can do nothing.
Blocker Question
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Can the US block Japan if there not at war?
The specific scenario is T3 And Japan has a navy and transports at the Caroline islands. The UK declared war on the Japanese on UK T2. The Japanese want to attack Kwangtung via amphibious assault, however there is a US destroyer is sea zone 21. So the question is can Japan move past the US destroyer and attack Kwangtung.
Thanks!
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Assuming that Japan and the US are not at war at that point in time, Japan can indeed do that.