You can also ignore naval ships of a power that you are not yet at war with. Say as Japan you are at war with the UK and not the US. If there are UK subs/transports with US surface war ships in the same sz, you could choose to ignore all the ships and bombard for an amphib assault.
I know that if Japan was previously attacked by the UK/Anzac (is at war) that it could ignore the US ships (not at war).
I do have one question concerning US not yet at war. Say Japan is still only at war w/China in the 2nd round. The UK/US have moved ships to say sz 43 (Borneo). Japan attacks the UK (unprovoked) in sz 43 immediately bringing the UK/Anzac into the war. This act is also considered a Dec of War w/US as well. Can Japan still ignore the US ships there, or does this attack also mean you must attack the US ships because your intent will bring the US into the war (like now having to declare a convoy attack on your turn). It will also make a difference if you can bombard Borneo as well if there are no UK surface war ships, only US surface war ships in the sz.
I’m sorry in advance, as I think this may have already been answered, but I missed it.
Edit:
I would think that an unprovoked Jap attack on the UK ships or amphib on Borneo would immediately make the US ships in that sz (and any other) hostile to Japan, and would defend.