What about this:
In 1950, during the Korean war, after McArthur outlaws the Japanese communist party at the peak of its popularity, Sanzo Nozaka, leader of the party, declares the need for a violent communist revolution in Japan. Plans begin to be set in motion.
In 1953, after the end of the occupation of Japan, and the Korea war, a massive communist uprising is launched, with attacks at numerous millitary and goverment locations. The revolt is suppressed on most of the country, but the communists succeed in taking over Hokkaido. The Japanese government lauches numerous attempted invasions but are kicked of the island time and again. The U.S, heavily demoralized from the Korean war, only sends material aid. The soviet union, China and even some north koreans send considerable aid to the communists. After a couple of years the war reaches a stalemate and all military action ends. The People's Republic of Japan is proclaimed.
The PRJ begins a major industrialization effort and mines the hell of the Hokkaido strait.
In 1960 Sanzo Nozaka dies and is replaced by Kenji Miyamoto, who begins to push the country out of the soviet sphere in a similar way to kim in Korea. He establishes a cult of personality a movement to expand the country population and to develop a strong navy. The society grows very authoritarian under his rule. After the fall of the Soviet Union, The United States sanctions a trade embargo on the PRJ (simply called Hokkaido by most). There's almost a war, the country becomes extremely isolated and famine ensues.
In 2007 Miyamoto dies and Akira Koike takes over as premier.
Today Hokkaido(or the PRJ as it calls itself) is completely closed of from the rest of the globe. It considers itself the last bastion of socialism in the east, besides it's sister republic in North Korea. Famine is still commonplace as well as a massive degree of repression. The current Chairman shows no interest in ending such practices