If Japan joined the Allies in June 1940

What changes if Japan avoids the Second Sino-Japanese War and joins the Allies on 25 June 1940?

American politics faces a severe contradiction, sanctions become harder to justify, and Japan can support Allied operations while Washington reassesses its Pacific posture.

Without the Second Sino-Japanese War, Washington loses much of the justification for sanctions while Japan appears to support the result America says it wants. American policy would then face the contradiction of resisting a cooperative Japan without the expansionist conduct that had justified pressure. The counterfactual distinguishes demonstrations of status from funded military readiness, with Pearl Harbor's exposed infrastructure as an example. A Japanese entry on the Allied side could also combine Royal Navy and Imperial Japanese Navy carriers against Italy, making a follow-up daylight strike after Taranto possible. The scenario changes both Pacific diplomacy and the early Mediterranean balance.