In the aftermath of the first pacific war, Japan was forced to pay massive reparations. Japan tried printing out more currency to pay off the debt, but this caused massive inflation. Emperor Herohito was executed to officially end the Japanese Empire. Anti-American sentiment was quite common in the Japanese republic and got even worse when Japan returned to its fascist roots in 1959 under Fuchi Kazuki. At that moment, Japan stopped paying reparations and the country became more militaristic. From 1965 to 1970, Japan took the following countries in order: Korea, Philippines, Indonesia, Singapore, Malaysia, Thailand, and all of former French Indochina. As a result, President John F. Kennedy put the country under sanctions. Kennedy also armed the Chinese with US weaponry in hope that they could hold back the Japanese.
In April 11 1971, Japan began its Chinese campaign. They invaded China through the nothern area and established a puppet state called "Manckuko" and the southeastern area. A blockade was placed to keep American ships from delivering goods to China. Tensions ran high between the US and Japan though the US did not enter the war. Japan signed an armistice with the US in May 5, 1974 guaranteeing no confrontation. This bought the Japanese time to leave China in disarray much to America's shock and dismay.
Later, a plan known as operation samurai was concocted by Fuchi Kazuki himself. Phase 1 involved all 20 of its I-500 submarines resurfacing near places with strong military activity and launching each of their 3 planes to first destroy any airfields then destroy other military targets. A few of them contained infected fleas with anthrax, measles, and even bubonic to infect soldiers. Phase 2 would see much of the japanese fleet finishing off american military positions in the west coast and the invasion of west coast states, hawaii, alaska, and the panama canal. Phase 3 would be the basic invasion of the US: the Japanese army would tear through the northeastern states as they had flat land and low population density followed by a campaign cutting through the great plains towards Washington D.C. While this was occurring, a japanese blockade would use its planes to harass american ships. This plan was designed to move quickly as japan, at around 100 million had the entire military age population (3/4 of their army would take the US) in its military while America had <1 million men but would quickly increase under a military draft. Many generals were worried about the plan but Kazuki didn't like opposition so they kept quiet.
The first phase was carried out on the Fourth of July 1975 and the second occurred the following day. The US was caught off guard and Japan made its advances through the northeastern states. However, the plan fell apart when the US used its airbases from the southwest to bomb japanese supply lines and cut off occupied Montana from the rest of the invasion force. With the superiority of ameirican air power, the japanese blockade was broken. Since the US had been working on their own submarine carriers in secret, they moved them to occupied california to catch the japanese off guard. This is followed by the american fleet which decimated japan's fleet as ground forces liberated the major coast cities. Operation samurai was an absolute failure 3 years after it was enacted. Since the japanese military's size was reduced dramatically, there were major revolts in other occupied territories as japan was unable to suppress them. The conflict ended when America dropped 2 atomic bombs on Hiroshima and Nagasaki In July 1978. Kazuki urged them to continue fighting but a mutiny occurred leading to Japan's unconditional surrender.
The US cut down on japan's military but they also rebuilt its infrastructure. To prevent another japanese uprising, massive reparations were not sought and Kazuki was executed. Japan became much more peaceful and made anime along with china.
The US also got more involved in world affairs, particularly the pacific area. They became close allies with China and Japan to this day.