The United States may not declare war on Japan unless Japan first declares war on it or makes an unprovoked declaration of war against the United Kingdom or ANZAC
Ok, there is the exact wording
By the spirit of the rules……If Japan makes an unprovoked declaration war on any allied power (other than China, with whom it is already at war) then the United States is “obliged” to declare war.
Upon digesting what you are asking for specificly, which has also illuded everyone else in this thread becuase you didnt offer an example…“can US units block Japanese naval movement if Japan only declares war on ANZAC and UK?”
My answer (and I “suspect” Kreigs) would be “no”, by the letter of the rules. If Japan only declares war on UK and ANZAC on its turn, then US units remain neutral UNTILL their combat move phase, when the US may then officaly declare war.
So what you want to do is move units through the phillipines sea zone after declaring war on the UK and ANZAC, and leave the US units there intact.