One of those watching the carnage was Eisenhower himself

From The Road to Little Dribbling: Adventures of an American in Britain by Bill Bryson. Page 159.

I returned to the car and drove a dozen miles along yet more slow but glorious roads to Torcross, a hamlet on a dramatic sweep of coastline overlooking Start Bay. To the north from here stretches a duney expanse called Slapton Sands, so similar to the beaches of Normandy that they used it for a dress rehearsal for D-Day in the spring of 1944. Amid great secrecy, thirty thousand American troops were loaded on to landing craft and taken out into the bay to practise coming ashore, but by chance nine German torpedo boats spotted the activity and cruised at will among them, blowing the landing craft out of the water with ease and causing all kinds of mayhem. No one from the Allied side, it appears, had thought to line up suitable protection for the exercise, so the U-boats were able to move about undisturbed.

One of those watching the carnage was Eisenhower himself. Nobody seems to know how many people died. Numbers range from 650 to 950 or so. An information board at Torcross says 749 American soldiers and sailors died. Whatever the exact figure, far more Americans were killed that night than died in the actual landing at Utah beach just over a month later. (Casualties were much higher at Omaha beach.) It was the most lopsided rout America suffered during the war, yet few people have ever heard of it because news of the disaster was withheld, partly for purposes of morale, partly because of the general secrecy surrounding the invasion preparations. What is most extraordinary is that the Germans, having chanced upon a massive collection of boats and men engaged in training exercises just across the sea from the Cherbourg peninsula, failed to recognize that an invasion of northern France was imminent.