The Pacific War really started long before Hitler's panzers blitzed across Poland. The civilian government of Japan had lost control of the military during the Great Depression, with the result that the Empire of Japan had become an aggressive imperialist power with designs on all Asia. Meanwhile the old foe, China, the central government of which before the 1930s was weak to the point of almost total dissolution was finally beginning to recover from a particularly devastating last century, and after the Xi'an Incident the ruler of the Kuomingtang, Chiang Kai'Shek, was induced to make peace with the communist Chinese and present a united front to oppose Japan, which was already encroaching into Chinese territory.
The result was a declaration of war in 1937, a war in which the Japanese would push deep in to the Chinese interior. However Japanese atrocities in China earned the disapproval of the international community, which was particularly important to the Japanese war machine as it was entirely dependent on foreign oil imports to continue the fighting in China. Attempts by the Western powers to discourage Japanese imperialism backfired spectacularly, however - when the American, Dutch and British governments embargoed Japan, rather than accept humiliation and withdraw from China, the Imperial military planned an attack on the Western powers, to secure the resources necessary to continue the fighting against the Chinese.
And so on December 7th, 1941, the Pacific war became a part of the greater world war when Japan launched a massive surprise attack on American, British and Dutch targets in the Pacific.
