Early on 28 April 1944, eight tank landing ships, full of US servicemen and military equipment, converged in Lyme Bay, off the coast of Devon, making their way towards Slapton Sands for the rehearsal.
So vital was the exercise that the commanders ordered the use of live naval and artillery ammunition to make the exercise as real as possible, to accustom the soldiers to what they were soon going to experience.
But a group of German E-Boats, alerted by heavy radio traffic in Lyme Bay, intercepted the three-mile long convoy of vessels.
The heavily-laden, slow-moving tank landing ships were easy targets for the torpedo boats which first attacked the unprotected rear of the convoy.
A series of tragic decisions – including the absence of a British Navy destroyer which was supposed to be escorting them, but had been ordered into Plymouth for repairs, and an error in radio frequencies – led to three of the tanks being hit by German torpedoes.
More loss of life was caused by lifejackets worn incorrectly by soldiers and the extreme cold of the sea which resulted in hypothermia.
The exercise that killed nearly 1,000 American servicemen was considered by US top brass to be such a disaster that they ordered a complete information blackout.
Any survivor who revealed the truth about what happened would be threatened with a court-martial.
The Allied commanders were concerned officers who went missing during the attack could have ended up in German hands, where they might reveal the Allied intentions for the D-Day landings.
The commanders even considered changing details of the operation.
However, the bodies of every one of those officers with “BIGOT”-level clearance, a codename for a security level beyond Top Secret, were found and the tactics of D-Day were deemed to be secure.
An article in the US Stars and Stripes magazine following World War Two said family members of the dead were given no information other than what was in the original message about the death.
The family of Gunner’s Mate 3rd Class Thanuel Shappard only knew he had died on 28 April 1944.
In the late 1980s, while watching a documentary about Exercise Tiger, his mother noticed the date was the same as her son’s death.
It was only after researching the exercise it was confirmed Mr Shappard had been aboard Landing Ship, Tank (LST) 531, which was torpedoed and sunk by the German E-boats.
Even before the military exercise, villages surrounding Slapton Sands had been evacuated, involving the clearance of 30,000 acres and 3,000 men, women and children by the end of 1943.
As local resident Ken Small pounded the beach along Slapton Sands in Devon 40 years later, little did he know that the discovery of shrapnel, military buttons, bullets and pieces of military vehicles would lead to an all-consuming mission to tell the world the story that had so long been forgotten…[ ]