thanks for the responses.  I really appreciate it!

Another follow up to my questions because I’m still debating this point with a friend:

If USA is not at war
if UK is at war
if USA naval units share a sea zone with UK units and Japan attacks

Then are these points correct:
1. Japan can attack the sea zone
2. UK and US naval units defend together
3. This is not a declaration of war by Japan against USA

or are these points correct:
1. Japan can attack the sea zone
2. UK naval units defend
3. ignore USA naval units in the battle.

Thank you for clarifying that!  I guess this is the only exception then to the general rule that if you attack a sea zone or land zone, all the defending units from all nations can defend together.

ok, so part 2 of my question, if the US moves Naval units into a sea zone shared with UK naval units AND the US is not at war yet, does this prevent Japan from attacking the UK fleet there UNLESS Japan declares War against US?

Can us and uk naval units share a sea zone before the US is at war with anyone?