The decision against partition seems to have been made around June 1945 by the United States.

Initially, planning assumed a post-surrender Japan with a role for the other Allied powers, and a zonal occupation was assumed, which helped to motivate British participation in the planned Allied invasions of Japan.

Rough plans for a zonal division were outlined in a meeting between the US ambassador and Stalin in Moscow in late May 1945, and Joint Chiefs planning continued to work on the basis of the zonal occupation with the US, USSR, UK and China occupying various parts of Japan, and Tokyo prefecture jointly.

However, from June the US decided that it would deny other states a controlling voice and presence in the occupation of the Home Islands for military reasons, even if still giving lip service support to Allied participation. Soviet demands for a presence in Hokkaido were denied, and high level planners were determined that an occupied Japan would involve divided control zones.

Only a limited area of Japan in the south of Honshu was placed under the jurisdiction of British forces, and Japan proper under US administration. This reflected US desires to ensure the occupation of Japan was on American terms and administration.