Writer Ben Macintyre has written extensively, and lucidly, about deception campaigns of WWI. Agent Zigzag, Operation Mincemeat, and Double Cross all capture the efforts made by the British and its allies. No individual operation swung it but there was an amalgamation of disinformation on various fronts. It’s hard to put a tangible outcome, other than intercepted messages did show the Germans were giving the disinformation due consideration.

I highly recommend any of Ben Macintyre’s books as they’re hugely accessible.

What is interesting is that for all the press D-Day gets, it likely did not change outcome with respect to Germany, but rather the Soviet Union.

By the time D-Day happened, Nazi Germany was already being relentlessly ground down by the Soviets. I think something like 90% of German casualties occurred along the Eastern front.

Without D-Day, I think the most likely outcome would have been the Soviet occupation of all of continental Europe.

> I think something like 90% of German casualties occurred along the Eastern front.

IIRC it was roughly 3-to-1, or 75%