As the crunch D-Day offensive loomed, American forces needed to ensure their best chance of cracking the Nazi stranglehold on Europe.
So a huge dry run was planned on an English beach selected because it was so similar to Utah Beach – the westernmost of the five Normandy landings taking place five weeks later.
Exercise Tiger on Slapton Sands, Devon, in April 1944 should have been a series of slick, bloodless drills.
But a string of blunders meant hundreds of troops were killed in a friendly fire horror – and the dry run later became a real battle as German E-boats attacked exposed vessels at Lyme Bay, killing 749.
The tragedies claimed over three times the Allied casualties suffered in the storming of Utah – the largest loss of American life since Pearl Harbor.
General Dwight D Eisenhower slapped a secrecy order on Exercise Tiger and survivors, many of whom would fight on D-Day, were silenced.
Woody Johnson, US ambassador to the UK, said: “For a long time, many people had no idea so many hundreds of American servicemen lost their lives on the coast of Slapton Sands as they rehearsed for the D-Day landings.
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“Those men did not die in vain. Their sacrifices paved the way for comrades to succeed on the beaches of Normandy and begin the liberation of Europe from Nazi tyranny.”
The practice assaults used real ammo as Eisenhower wanted troops hardened up.
But a timings update on April 27, the day after the first drill, did not reach all vessels – and a wave of troops came under fire upon landing. It was rumoured in the fleet that as many as 450 were killed.
Then, at 2am on April 28, eight tank-landing ships converging at Lyme Bay, en route for Slapton Sands, took fire from E-boats.
The Nazis were alerted to heavy radio traffic and intercepted the three-mile convoy, hitting three ships with torpedoes.
A British destroyer meant to escort the convoy was still in Plymouth for repairs. But as typists had entered the wrong radio frequency, the US ships were on a different one and unaware.
Seventy-five years on, bootprints stretch across the beach in a poignant piece of sand art for a ceremony of remembrance. Each pair represents one of the 749 killed.
Among those honoured was Sergeant Louis Archer Bolton of Iowa. He was on LST 531 when it took direct hits.
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Niece Laurie joined the event yesterday and said: “Only five men from his platoon survived Exercise Tiger. He was just 19 and a newlywed.”
His story would never have been told if not for late Devon hotelier Ken Small, who fought for decades to ensure relatives heard the truth.
Ken’s son, Dean, said: “I’m proud of what my dad did. When I look at what he achieved and hear how much it means to people, it chokes me up.”
Ken moved to Torcross from Grimsby in the late 1960s and ran a guesthouse.
During an illness, he walked at Slapton Sands to recuperate. “He began to notice unusual objects on the tide-line,” said Dean.
“There were tunic buttons, bullet cases, shrapnel. He asked around and was told the villages had been evacuated for wartime exercises.”
Ken’s talks with locals confirmed letters were sent ordering people to leave. Pam Wills, 85, was 10 when Exercise Tiger was staged near her home in Strete.
She says: “The US soldiers talked to us. We liked them, they’d give us sweets and comic books. But they then disappeared.”
Pam remembers “explosions and gunfire” and her father, a watchman in the Royal Observer Corps based at Start Point, would see ambulances come and go so they “knew something was wrong”.
Allied commanders were concerned D-Day plans could end up in Nazi hands. But the bodies of every officer with top-level clearance were found.
Any survivors who revealed the truth would face a court martial, locals were ordered to stay quiet and victims’ relatives were kept in the dark.
That was until Ken started unravelling the story – even unearthing an American tank from the seabed.
Ken, who died in 2004, campaigned to raise the tank and make it a memorial, spending years talking to the US government, veterans and members of the E-boat crews that hit the ships.
He achieved his goal in May 1984 and as soon as the tank was hauled out of the water, the story spread internationally.
On the 75th anniversary of the disaster, families and dignitaries gathered around the tank and roses will be scattered in Lyme Bay next week.
Artist Martin Barraud, who created the There But Not There sand art for the First World War centenary, hopes the bootprints work will help people “see, hear and feel what happened”.
