A 1939-40 Pacific war before the Kido Butai matured

What would a Pacific war look like if Japan attacked the United States in 1939 or 1940?

Japan would fight before the Kido Butai matured, while early American and British entry would create a very different alliance structure and wider war.

The carrier striking force recognised from the 1941 campaign did not yet exist as a mature formation in 1939 and was still developing in 1940. An early attack would therefore confront the United States before Japan could employ the same integrated six-carrier method used at Pearl Harbor. Britain would probably join the Pacific conflict immediately, widening a European war that had only just begun. Italian and German choices would be less predictable because the later Axis alignment with Japan was not yet in place, making the counterfactual as much about alliance timing as naval strength.