Why the Anglo-Japanese Alliance unraveled in 1921

The chronology used here treats 1921 as the point when the Anglo-Japanese treaty technically ceased to operate, although its formal termination came later. American pressure contributed to the alliance's decline. Britain was also increasingly concerned that Japan had become a capable naval state rather than only the useful army power imagined at the alliance's beginning. Britain had a long record of cooperating with strong continental militaries while worrying about fleets that could contest its maritime position. The break was gradual, with the simultaneous Sempill Mission showing that professional respect and technical cooperation continued as the strategic alliance weakened.