Even an unoccupied Dutch Territory?
This could be better clarified in the FAQ with another sentence.
What brings each power into the war is described in their section. There is nothing in the US section that says the US is allowed to declare war on Japan, just because Japan attacks a Dutch territory, so they can't. However, in order to attack a Dutch territory Japan must be at war with the UK/ANZAC. If Japan was the one who declared first, the US can declare war anyway, attacking the Dutch doesn't matter. If the UK/ANZAC declare first, the state of war that allows Japan to attack the Dutch exists, but none of the things which allow the US to declare war have occurred, so they aren't brought into the war.