TOKYO -- In prewar Japan, the emperor was absolute. Abdication was forbidden because it would create two absolute figures in the same era, each possibly taking away from the other's authority.
This issue arose again after the war in the debate on a new Imperial House Law. The Ministry of the Imperial Household -- since replaced by the Imperial Household Agency -- opposed allowing abdication in light of the question of Emperor Hirohito's responsibility for the war. Following deliberations in parliament, the law was written to allow succession of the throne only after a reigning emperor's death.