Welcome to the forum Jman.
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.
Assuming that Japan and the US are not at war at that point in time, Japan can indeed do that.