As a victor in WW2, Japan, like America, Brazil and Iran, was elevated to superpower status. In order to consolidate their position in South East Asia, Emperor Hirohito suggested that the East Asian Co-Prosperity Sphere (Hitler is an American---Version 1) be set up. China was the first to join, since they saw it as repaying a debt for Japan helping them defeat communist insurgents. Eventually, most of South East Asia and Oceania would be under this bloc.
Today, Japan is one of the most technologically advanced countries on the planet, with the best healthcare system in the world, although the Kobe earthquake of 1995 would test its capabilities. Japan is also known for its relatively low crime levels, which are third only to America and Germany. however constant problems with resistance movements in Vietnam, Australia and New Zealand, including the Oceanian uprising of the Late 60's, when all three countries revolted at the same time, have been quite a headache for the Japanese military (but to many Japanese recruits, a chance to prove themselves in battle)
Both Australia and New Zealand were granted Home Rule (like Ireland OTL)in the mid 1980's with their own respective parliaments, and they established treaties with the US to solidify that independence.
The Occupation of Vietnam has been the greatest headache for the Japanese, with over 200,000 dead since the occupation began. the Vietminh have been covertly supplied by the Russians and the Germans, who have sustained them. many have proposed abandoning Vietnam, but its resources are too valuable to abandon, so the casualties are grudgingly accepted by the Japanese high command.
During the Cold war, the Japanese and Americans got their first man on Mars by Emperor Akhito's birthday on December 23, 2007.