people call the turning point in the pacific to be The battle of Midway.
That was a stunning victory however...at the end of the battle the Japanese were still a very capable force...they still had several fleet carriers and several escort carriers. a couple were being built..Japan had the 2 largest battleships ever built... and Japans Cruisers and destroyers were still something not to fool around with.
Not until we decisively ejected Japan from The Solomon islands in 1943 thus completing our first offensive against Japan was the idea secured that this war would be won. After Japan couldn't effectively defeat us in the many carrier/warship battles or land battles with troops that took place we knew that we would only be getting stronger and as we got stronger the enemy only weaker.
Once it was a fact that we could defeat Japan at sea, in the air, and on land was total victory finally assumed for the US.