Thanks to some pointers from @LateralFractal I was able to find the answer to my question.
The role of japans emperor as a symbolic head of state is one that dates back to around the 10thC. Since the 6th century the political system had imitated a Chinese system in which power was designated to the emperor according to the meritocratic model. However in practice this this model had become defunct and the powers of the emperor had been slowly devolved to parliament members.
This casual agreement was later revoked in 1868 with the Meiji Constitution. According to Wikipedia this was defined in an effort to socially reform Japan and strengthen it to the level of other western nations.
After the Meiji Restoration, which restored direct political power to
the emperor for the first time in over a millennium, Japan underwent a
period of sweeping political and social reform and westernization
aimed at strengthening Japan to the level of the nations of the
Western world. The immediate consequence of the Constitution was the
opening of the first Parliamentary government in Asia.
The Meiji Constitution established clear limits on the power of the
executive branch and the Emperor. It also created an independent
judiciary. Civil rights and civil liberties were guaranteed, though in
many cases they were subject to limitation by law. However, it was
ambiguous in wording, and in many places self-contradictory. The
leaders of the government and the political parties were left with the
task of interpretation as to whether the Meiji Constitution could be
used to justify authoritarian or liberal-democratic rule. It was the
struggle between these tendencies that dominated the government of the
Empire of Japan.
However the Meiji Constitution was suspended by allied forces when Japan surrendered in World War 2, they deprived Japan of sovereignty, the power of an authoritarian monarch. I can only assume this was under the impression that by reducing powers of the monarchy they would avoid further conflict in the future. This was drafted into the constitution of Japan
The constitution provides for a parliamentary system of government and
guarantees certain fundamental rights. Under its terms the Emperor of
Japan is "the symbol of the State and of the unity of the people" and
exercises a purely ceremonial role without the possession of
The constitution, also known as the "Postwar Constitution" ( 戦後憲法
Sengo-Kenpō?) or the "Peace Constitution" ( 平和憲法 Heiwa-Kenpō?), is
most characteristic and famous for the renunciation of the right to
wage war contained in Article 9 and to a lesser extent, the provision
for de jure popular sovereignty in conjunction with the monarchy.
The constitution was drawn up under the Allied occupation that
followed World War II and was intended to replace Japan's previous
militaristic and absolute monarchy system with a form of liberal
democracy. Currently, it is a rigid document and no subsequent
amendment has been made to it since its adoption.
Ergo, the current system is one devised by an international coalition and, although related to an original model, not one that was defined by the Japanese. Very interesting!