Which was a greater turning point in American history, the D-Day invasion or the bombing of Pearl Harbor?
Pearl Harbor brought the United States into the war. Without the United States, there would not have been a D-Day. So while both events were turning points in the war, Pearl Harbor was the foundational of the two, as the second would not have happened without the first.
Without question, Pearl Harbor was the bigger turning point. Pearl Harbor caused something completely new and relatively unexpected to happen. D-Day was "just" a step along the way to something (the fall of Germany) that was quite likely to happen anyway.
Pearl Harbor forced the US into WWII. No matter what President Roosevelt wanted, the US was not at all likely to have gotten into the war absent some sort of attack like this. Therefore, Pearl Harbor pushed the US, in one fell swoop, into a war it did not want. This means it was much more of a turning point. The people who fought in the D-Day invasion were undoubtedly heroic, but the battle itself was not a major turning point in the overall history of the US.
I would have to answer that the bombing of Pearl Harbor was the greater turning point in United States history regarding entering World War II. Japan's bombing of Pearl Harbor on December 7, 1941 caused the United States to declare war on Japan thus causing Italy and Germany to then declare war on the United States causing the United States to enter the war in both Europe and Asia. This act directly resulted in the United States troops landing in Normandy, France on June 6, 1944 on what is known as D-Day.