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The text below gives the bare facts:

In late 1943, as part of the war effort, the British Government evacuated approximately 3000 local residents in the area of Slapton, South Hams District of Devon. Some of them had never left their villages before.

Landing exercises had started in December 1943. Exercise Tiger was one of the larger exercises that would take place in April and May 1944. The make up of Slapton Beach was selected for its similarity to Utah beach, namely a gravel beach, followed by a strip of land and then a lake.

The exercise was to last from 22 April until 30 April 1944, at the Slapton Sands beach in Slapton, South Devon. On board nine large Tank landing ships (LSTs), the 30,000 troops prepared for their mock beach landing.

Protection for the exercise area came from the Royal Navy. Two destroyers, three Motor Torpedo Boats and two Motor Gun Boats patrolled the entrance to Lyme Bay and Motor Torpedo Boats were watching the Cherbourg area where German E-boats were based.

The first practice took place on the morning of 27 April. These proceeded successfully, but then early in the morning of 28 April, German E-boats that had left Cherbourg on patrol spotted a convoy of 8 LSTs carrying vehicles and combat engineers of the 1st Engineer Special Brigade in Lyme Bay and attacked. One of these E-Boats was S-130 now in dry dock in Plymouth UK. One transport caught fire and was abandoned, a second sank shortly after being torpedoed, a third was set on fire but eventually made it back to shore. The remaining ships and their escort fired back and the E-boats made no more attacks.

The attack caused over 600 casualties, compared to only about 200 in the Utah Beach invasion. 638 servicemen were killed – 441 U.S. Army and 197 U.S. Navy personnel. Many servicemen drowned in the cold sea waters while waiting to be rescued. Soldiers unused to being at sea panicked and put on their lifebelts incorrectly. In some cases this meant that when they jumped into the water, the weight of their combat packs flipped them onto their backs, pushing their heads underwater and drowning them. Dale Rodman, who travelled on LST 507, commented „The worst memory I have is setting off in the lifeboat away from the sinking ship and watching bodies float by.“

Of the two ships assigned to protect the convoy, only one was present. HMS Azalea, a corvette was leading the nine LSTs in a straight line, a formation which later drew criticism since it presented an easy target to the E-boats. The second boat which was supposed to be present, HMS Scimitar, a World War I destroyer, had checked into Plymouth for minor repairs. The American forces had not been told this. When other British ships sighted the E-boats earlier in the night and told the corvette, its commander failed to tell the LST convoy, assuming incorrectly that they had already been told. This did not happen because the LSTs and British naval headquarters were operating on different frequencies. Also, British shore batteries defending Salcombe Harbour had seen silhouettes of the E-boats but had been instructed to hold fire so the Germans would not find that Salcombe was defended.

When the remaining LSTs landed on Slapton Beach, the blunders continued. The British heavy cruiser HMS Hawkins shelled the beach with live ammunition, following an order made by General Eisenhower, the Supreme Allied Commander, who felt that the men must be hardened by exposure to real battle conditions. British marines on the boat recorded in its log book (the only log which has since been recovered from any of the boats) that men were being killed by friendly fire. „On the beaches they had a white tape line beyond which the Americans should not cross until the live firing had finished. But the Marines said they were going straight through the white tape line and getting blown up“.

Aftermath
As a result of official embarrassment and concerns over possible leaks just prior to the real invasion, all survivors were sworn to secrecy by their superiors. Ten missing officers involved in the exercise had Bigot-level clearance for D-Day, meaning that they knew the invasion plans and could have compromised the invasion should they have been captured alive. As a result, the invasion was nearly called off until the bodies of all ten victims were found.

There is little information about how exactly individual soldiers and sailors died. Various eyewitness accounts detail hasty treatment of casualties and unmarked mass graves in Devon fields.

Several changes resulted from mistakes made in Exercise Tiger:

Radio frequencies were standardised; the British escort vessels were late and out of position due to radio problems, and a signal of the E-boats‘ presence was not picked up by the LSTs. Better life vest training for landing troops.
New plans for small craft to pick up floating survivors on D-Day.The casualty statistics from Tiger were not released until August 1944 along with the casualties of the actual D-Day landings themselves.

There is still very little documentation in official histories about the tragedy. Some commentators have called it a cover-up, but the initial critical secrecy about Tiger may have merely resulted in longer-term quietness. In his book The Forgotten Dead – Why 946 American Servicemen Died Off The Coast Of Devon In 1944 – And The Man Who Discovered Their True Story, published in 1988, Ken Small declares that the event „was never covered up; it was ‚conveniently forgotten'“. Charles B. MacDonald, author and former deputy chief historian at the U.S. Army Center of Military History, notes that the incident was reported in a press release issued from the Supreme Headquarters, Allied Expeditionary Force, and appeared in the July issue of Stars and Stripes. In addition, the story was detailed in at least three books at the end of the war, including, Captain Harry C. Butcher ’s My Three Years With Eisenhower (1946), and in several publications and speeches in the intervening years. MacDonald surmises that the press release went largely unnoticed in light of the larger events that were occurring at the time, the battle for France in the summer of 1944, and the fact that they were just glad that the war was over in 1946.