First time this has come up for my group:
J1: Japan player does not declare war against the U.S. Japan consolidates its transports into a heavily defended sea zone.
US1: US player sends the Filipino DD to the area with the Japan super-fleet (including the transports).
Now, if Japan declares war against the U.S. on J2 the transports cannot be used in that turn, or can they? Or ruling was that if Japan declares war, and it shares a seazone with the power they just declared war upon - then that superfleet is technically locked down in combat and must resolve the combat during its turn. Since the trannies were (technically) part of the combat where the Japan superfleet will annihilate the US DD they cannot move during the noncom move (or, for that matter, perform an amphibious assault that turn).
Is this legal?
It was an interesting move - because it dared the Japanese to declare war against the US, and if so, it hampered the movement of their important transport assets. It also kept Japan from invading the Phillipenes on J2 (which was their plan) and forced them to wait until a much less-optimal J4 for the invasion (they still took it, in case anyone was wondering) after spreading out their transports to various seazones.