Britain and the British Commonwealth were involved in the war in the Pacific from the same moment as the United States. The attack on Pearl Harbour coincided with a Japanese invasion of Malaya, a British possession at the time. British and Commonwealth troops bore the brunt of the Japanese expansion into South East Asia, the worst defeat coming with the fall of Singapore in February 1942.

The British were also fighting Japanese advances towards India in what is known as the Burma Campaign. This involved British troops and large numbers of Indian troops. This tied up quite a bit of Britain’s resources in the region with the rest trying to desperately but futile hold back the Japanese invasions of Britain’s other colonial possessions in the region.

The Australian forces fought throughout the Pacific war, preventing the Japanese from capturing Port Moresby in Papua New Guinea and eventually pushing them back over the Owen Stanley mountain range in what is known as the Kokoda Trail Campaign. US forces under MacArthur took over most of the fighting after 1942 and undertook the majority of operations in the Pacific during MacArthur’s drive back to the Phillipines. Australian forces did take part in some of those operations, notably the Borneo Campaign in the final days of the war.

You also have to remember that China had been fighting the Japanese longer then anyone and was doing it at the same time it was fighting its own civil war despite an agreement between the Nationalists and Communists that they would set aside their quarrel until the Japanese were dealt with. So you can see that the United States was far from alone in fighting the Japanese and while the Us certainly played an enormous role in the war in the Pacific, the majority of Japan’s army was deployed in China with only a relatively small portion conducting operations in areas where they fought the US. That doesn’t include naval power however which the Us engaged and destroyed the bulk off between 1942 and 1945.