Where was the first major Japanese defeat during World War 2?

The Japanese lost 4 aircraft carriers and other ships at the Battle of Midway (Midway island) just 7 months after Pearl Harbor, from June 4 to June 7, 1942.

Although the Japanese were the tactical winners in the much smaller Battle of the Coral Sea (May 4-8, 1942), it was a setback in terms of the continuing expansion of Japanese control of the Pacific. Both the US and Japanese had a carrier sunk.