There is much debate whether or not Hirohito made the declaration. There is a lot of controversy whether he had actually done it or was it his government that was the one who was actually pulling the strings since he didn't care much about what was happening in China and his government and his cabinet took care of all Chinese invasion.
The purpose of the declaration was to justify imperialistic tendencies. They had to officially declare that they were participating in the war so they would get international support from their allies such as Germany in order to spread over China and the South East Asia region which was mostly colonized by European forces.
