The USA carried a larger share of the World War II burden against Japan, as compared with the European theater, where it was more broadly shared. Much less well known is that its three most important allies in that fight are most often ignored: China, India (though still under British rule), and the Soviet Union. China kept the largest Japanese army forces tied down there for the duration of the war, where the Japanese suffered heavy casualties and Japan bore heavy costs, even though Chinese forces were unable to liberate much of their country during the war.
The Soviet Union did not enter the war against Japan until its final days, so it is usually the most ignored of the US allies against Japan, but when it did enter, it did so decisively, with such massive veteran forces transferred from Europe, where they had beaten Germany, that Red Army was able to blitz right through the largest intact Japanese army, the Kwantung Army in northeast China, in mere days, reaching as far as northern Korea. Japanese army records show that this rapid and irresistible destruction of one of its last major intact armies convinced the Japanese ARMY, specifically, that the war was over for them. The army high command feared that if Japan did not surrender immediately to the Americans and invite occupation by them, the Soviet Union would invade northern Japan (beyond southern Sakhalin and the Kuril Islands, which Russia did quickly conquer and still holds today) and create a Soviet regime there. Japan’s top generals surrendered because they were more afraid of the Soviets than of the Americans. Japan did not want to end up divided by the coming Cold War, as Germany already was and Korea soon would be. Thus the USSR, not the US atomic bombing of Hiroshima and Nagasaki, which the Japanese army sneered at, saved the USA from having to invade Japan itself (other than Okinawa)
The most important US ally in the Pacific theater itself was Australia, though as a much smaller country than these three, its contribution against Japan was smaller overall. Britain also helped, but almost exclusively along the Burma-India border (using mostly Indian troops, credited above) after the February 1942 fall of Singapore, until the final months of the war, when the Royal Navy finally sent a major fleet to the Pacific, but by then the war was largely won. Britain was preoccupied by the war against Germany and Italy. When the British did finally arrive in force in the Pacific, they were more concerned with restoring their empire, and that of their French ally in Indochina (including the newly-independent Vietnam), than defeating Japan. The weak Dutch naval and army forces in Indonesia (then Dutch East Indies) were also initially engaged against the Japanese, but defeated during the initial Japanese offensive in the opening months of the war, as were the Filipino forces fighting under US control.