There were many significant turning points in ww2, within which, had they not happened as they did the outcome of the war could, arguably, have been very different.
In particular the attacks on Pearl Harbour, as they changed the whole face of the war. If the Japanese had not attacked Pearl Harbour in 1941, then the war in the Pacific may have never escalated to the use of nuclear war fare.
It is important to note however, it can also be argued that the dropping of the atomic bombs was the biggest turning point of the war. This can be inferred as it ultimately changed modern war fare forever.
Despite this, if the Japanese hadn't attacked Pearl Harbour they may have never provoked the USA