After losing WWII, Japan was occupied by General Headquarters (GHQ). Early idea from US was to set ‘demilitarized democracy’ upon Japan, eternally forbid war capability. However due to the beginning of the Cold war, direction was changed into ‘remilitarization and economic recovery’ to defend Soviet. In June 25th 1950, Korean War begun and idea became to reality. There was changing paradigm of security awareness around Japan, President Truman treat Korean War as possible threat upon pacific region and US. Also, he understood that forfeit Korean peninsula means another compromise - It could extend toward Japan especially Soviet wanted to separate Japanese territory after WWII.
General MacArthur, who opposed Truman’s idea of military react in Asian region also shared same consideration during Korean War. He also agreed that same scenario could happen around Japan, even in the west coast of US. Before Korean war, MacArthur disagreed with changing policy of Japan occupation into cold war settings - especially Pentagon’s NSC 13/2, criticized it. However, situation aroused and MacArthur ordered to establish national Police Reserve (Constabulary) for security, due to the absent of US military dispatch toward Korean peninsula. Prime minister Yoshida of Japan accepted and found constabulary without discussion in parliament. As followed, US chose to act ‘generous compromise’: resigned rights for damage claims in peace treaty with Japan, abolished remilitarization / industrialization limits and promised full independency without directing groups. With possible threats of war and security, Japan signed for remilitarization and revised peace treaty. Thus, Japan remained as ally of US rather than conciliation of Asian region.