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As long as the US player complies to this rule (from Alpha2):
“In the Pacific it can only end naval movements in sea zones that are not adjacent to Japanese-controlled territories and islands.”
So you are allowed to bring additional units there. But the journey might be tricky - and maybe you don’t have enough time to do so.
However, are you really playing Alpha2-rules, still? I mean those have been obsoleted since autumn 2011, when Alpha3 had been issued, the predecessor of the (current) Second Edition ruleset.
In the current rules, it reads: " … while it’s not at war with Japan, the United States may not … end the movement of its sea units in sea zones that are adjacent to Japan-controlled territories. "