Japan is one of the three great powers that dominate the world. Though it is less powerful than the Greater German Reich, it is still more powerful then the United States.
The Japanese entered the war on February 7th, 1943, when it attacked and then invaded Hawaii. It eventually won the war, although it couldn't permanently solidify their position in the Pacific, especially with Donald Rumsfeld openly challenging Japanese hegemony in the Pacific.
The Axis split would be considered the key moment. Many say it all began with the Japanese alarm over the Von Braun MkII. Others would say with the Honduras missile crisis or the Nordland civil war.
The Japanese Empire rules most of the Pacific and SE Asia. All the native populations were used as slave labour (the Japanese had far less interest in eugenics than the Nazis) rather than killed outright, mainly so Japan could exploit the abundant resources at their disposal.
Following the victory in the Pacific, the Japanese Army has fought a long, bloody and at times expensive anti-insurgency campaign against various Communist, pro-Soviet and anti-Japanese guerrilla and resistance movements, with the heaviest fighting in China and Indochina. With Peak Oil crippling their economy, Japan has tried various attempts to harness alternative energy sources.