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When Japan bombed Pearl Harbor - 7 December 1941.
Before that it was potentially avoidable, after it there was no chance.
Don't agree about Germany having to declare War - The Empire of Japan declared war on both the British Empire and the USA with the attack on Hawaii. America was allied to USSR and UK as a result of this act of war.
Note: it was not necessary for all Allies and all Axis powers to be at war - USSR did not declare War on Japan until August 1945 (after Hiroshima) - but nobody would dispute they were not involved in WW2
Hope this helps...