In January 1939, Japan has not yet signed the Tripartite Pact, French Indochina is unavailable as a Japanese base, and Germany or Italy would not automatically fight for Japan. That changes the likely coalition, the basing problem, and the diplomatic incentives for Britain, France, the Netherlands, Germany, Italy, and the United States. Hitler and Mussolini might even support the anti-Japanese coalition to gain prestige, recover Tsingtao, or legitimize their own military expansion. Diplomacy and geography therefore shape the counterfactual before the carrier air groups enter the calculation.
Chapter · 5:11-10:19
January 1939 changes the alliance map
The later Axis alignment cannot be assumed in January 1939: Germany and Italy could pursue prestige and rearmament opportunities on Britain's side.