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It is generally considered that the Pacific War began on 7/8 December 1941, on which date Japan invaded Thailand and attacked the British possessions of Malaya, Singapore, and Hong Kong as well as the United States military bases in Hawaii, Wake Island, Guam and the Philippines. Some historians contend that the conflict in Asia can be dated back to 7 July 1937 with the beginning of the Second Sino-Japanese War between the Empire of Japan and the Republic of China, or possibly 19 September 1931, beginning with the Japanese invasion of Manchuria. However, it is more widely accepted that the Pacific War itself started in early December 1941, with the Sino-Japanese War then becoming part of it as a theater of the greater World War II.[nb 9]

When do most people believe the Pacific War began?
early December 1941