Here's the best I got.
When the Republicans take back control of Congress in 1946, isolationist forces prevail rather than the internationalists, and they gut American occupational spending in Japan. The situation deteriorates there, with Japanese Communists growing in number and influence, and though both Truman and MacArthur call for back up from the Congress, it never arrives. Bob Taft defeats Truman in 1948 and finishes pulling American troops out of Japan, which is returned to local governance in 1950.
A General Election brings the Japan Socialist Party to power, though its left flank and communist agitation keeps it from governing well, allowing the conservative forces to regain control, although with little precious support from the country, which is still largely poor and starving thanks to a generous lack of infrastructure. While the conservatives heavily invest in infrastructure and try to bring the economy back on a stable footing, they are unable to defeat the growing Japanese Communist Party, which organizes within the labor movement and other sectors, such as the agricultural sector. Intellectuals join in the fray in the sixties, when, as an economic shock sends the Japanese economy into a spiral, the campuses and factories revolt.
The Japanese Revolution of 1968 spirals out of control quickly, with various communes established in major cities and with the Japanese Self Defense Forces doing their best to maintain order. A state of emergency is declared, though the incumbent government is largely seen as impotent to protect the nation from the growing unrest and the activity of the commune governments.
A rousing speech by Yukio Mishima, a former author and actor leads to a coup d'etat against the incumbent government in 1970, with the restoration of the powers of the Emperor on paper, but in reality, the imposition of what is termed the 'Mishima Shogunate' by the foreign press. Mishima reorganizes the Self Defense forces into the Imperial Japanese Army and crushes the communes, bans the Japanese Communist Party, and liquidates it's supporters.
By 1971, order has been mostly restored. Mishima, now Prime Minister of Japan, begins to implement a rigidly reactionary social program to return Japan to it's 'founding values', which loosely translates to turning Japan into a reactionary state of Emperor worship and state Shintoism. The last vestiges of Japanese democracy are abolished and society is 'reformed' in the seventies and eighties, all the while Mishima also promotes the development of 'egalitarian capitalism', i.e. state capitalism, to compete with the Soviet Union, now the world's largest economic power thanks to the rise of American isolationism, and with the closed American economy. The 'Japanese miracle' dominates literature of the decade, and with the collapse of the United States in a socialist revolution in 1989 and the eventual rise of communism in Latin America, Japan assumes the leadership of the 'Free World' (however distorted this now coalition of fascist and theocratic states has become) and begins a military build up to challenge the power of the Soviet Union.
Mishima dies in 1995, and is succeeded by a close family member. As the military build up continues, the Japanese Committee of Public Safety (the de facto ruling commission of Japan) announces in 2006 that it has acquired nuclear weapons and that it shall produce nuclear weapons to contain communist influence in the Pacific. The Soviet Union is not amused, and tensions continue to rise between the two powers, with a 'Second Russo-Japanese War' continually predicted by pundits every other week...