Question

Is the answer Midway?
U.S. 1st major offensive in the pacific, from this point on the Japanese were on retreat.

Answer 1

One word. Guadalcanal. Look to what happened there and you'll find your answer.

Answer 2

I think it was the battle of Coral Sea. It helped stem the Japanese from capturing Australia and stemmed Japanese Carriers from dominating Pacific. The battle was the first real carrier battle without hte ships sighting each other, fought wholly with planes