After the French army was more or less defeated in Europe and many troops were brought there, the Indo-Chinese area in Asia was isolated and not well protected anymore. The Japanese Emperor uses this chance to claim this area for his own. Because Japan and Russia had a Non-Attack-Pact the Japanese weren't afraid of protest from the Russians. They occupied Vietnam on the 28th of July and on with the permission of the Dictator of Thailand they controlled also Thailand. From this basis they attacked the British colonies Burma and Malaya.
On the 7th December the Japanese airforce destroyed the American fleet in Pearl Harbour and therefor they declared war with the Americans. Now the Japanese concurred very quickly most of the Islands in the West Pacific. Only the Wake Islands and Hawaii were left to the Americans.
In summer 1942 the Japanese ruled over 450 million people in the whole west pacific. They helped defeating the European colonists and to achieve independence.
In august 1942 the American and Australian offence began. Under General McArthur the Americans freed the whole south pacific and Admiral Nimitz freed the northern part. The final defeat of the Japanese were the atomic bomb drops on Hiroshima (6.8.) and Nagasaki (9.8.). The Japanese Emperor surrendered and the peace treaty was signed on the 8.9.1951 in San Francisco.