Complex sentence: After the dissolution of the Empire of Japan and the establishment of the State of Japan on 2 September 1945, the ultranationalist societies were disbanded and socialism was decriminalized, ruling structure was changed from Tennō to the US-Japan alliance.
Split into simpler sentences: After the dissolution of the Empire of Japan and the establishment of the State of Japan on 2 September 1945, the ultranationalist societies were disbanded and socialism was decriminalized. Thus, ruling structure was changed from Tennō to the US-Japan alliance.