Why an Anglo-Japanese alliance could draw America into the war

If Japan joined Britain against Germany and Italy, would the United States still enter the war?

Probably yes; Washington could not comfortably allow Britain and Japan to win together and dominate the post-war order without American participation.

An Anglo-Japanese victory would create military ties, shared prestige, and a powerful imperial partnership. Remaining neutral would leave the United States with little influence over that alliance or the institutions built after the war. American entry would probably follow soon after Japan's declaration, perhaps once diplomatic intercepts reveal Tokyo's move. Participation would let Washington shape the coalition and prevent Britain and Japan from becoming the uncontested architects of victory.