The Battle of the Midway in June 1942, when the Japanese Navy lost most of its fleet of aircraft carriers and with it, Japanese supremacy in the Pacific Ocean.
What was the turning point of the war for the Japanese in WW2? Which battle in your opinion changed the course of the war?
Of course, the Japanese actually lost the war when they attacked Pearl Harbour with no plan "B". How did they ever think that the yanks would not take that rather seriously? A bit like the decision to bomb London by the Germans... Bad Mistake.