The no-attack scenario changes geography as well as politics. If Japan fought only the United States, neutral British, Dutch, and Australian governments would not automatically open their bases to American forces. United States submarines would then have to operate without the forward facilities that historically supported access to Southeast Asian waters. A bases-for-aid bargain was conceivable, but granting it could pull Britain into a second war at a moment when Churchill still had to weigh the price of confronting Japan.
Notable Point · 1:54:46-1:56:54
Neutral bases could restrict American submarines
If Britain, the Netherlands, and Australia stayed outside the war, American submarines would lose the forward access needed to reach much of Japan's resource network.