Japan cannot use French Indochina as an uncontested base for its later southern campaign because entering it in September 1940 means war with France and Britain. That removes the fifteen-month period in which Japan historically built resources and positions there before attacking south. The scenario proposes that the United States might join an early conflict to prevent Britain from emerging with overwhelming influence in Asia. Even if American ground forces initially remain in the Pacific, belligerency would sharply increase material support reaching the European war.
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French Indochina blocks Japan's historical buildup
A fighting France denies Japan its 1940 occupation of Indochina and may trigger an earlier Allied-American confrontation.