Let's say the US didnt pressure the UK to terminate the Anglo-Japanese alliance in 1923 and Anglo-Japanese relations grew ever stronger in the 1930s to the point where Britain actively supported the second Sino-Japanese War and provided Japan with help during it. What would happen when it came to WW2, no doubt Japan would be hugely strong with the support of the Royal Navy, the largest navy on earth and the British and Indian armies in India, Burma, Australia, New Zealand and the other British Asian colonies could it be Britain and Japan really would dominate the Pacific, i dont think the US would be able to beat a joint Japanese Empire-British Empire force, America only beat Japan because they got lucky at Midway and because British India and resistance forces kept half of the entire Japanese Army busy in Burma and other former British and Western European colonies.
Since Britain would be allied with Japan they would also be sort of allied with Nazi Germany, that would mean Hitler wouldn't be trying to invade so Britain wouldn't have to withdraw forces from Asia and the Pacific to fight the Nazis in Europe.
Could the Japanese dream of the "Great Co-Prosperitive Sphere" have been realised, would it be the "Anglo-Japanese Co-Prosperitive Sphere", would Britain and Japan dominate the pacific? Could the Japanese flag fly above the White House?