Sixty-eight years ago today, the Allies launched a massive dress rehearsal for the invasion of Normandy — the famous D-Day landings that would happen five weeks later. But that rehearsal turned into one of the war's biggest fiascos.

It took place on Slapton Sands, a beach in southwestern England. British historian Giles Milton wrote about the rehearsal on his blog last week.

"The beaches there are long and they're wide, so it gave the soldiers plenty of opportunity to really experience what it was going to be like," Milton tells weekends on All Things Considered host Guy Raz. "The beaches in the west of England are almost identical to the beaches in Normandy."

The rehearsal was given the code name Operation Tiger. The plan: To get landing boats into the English Channel, then have them simulate a water landing on the beaches of the Devon coast. The man in charge was the great Allied commander Dwight D. Eisenhower.

"He wanted to put them out in the rough waters of the channels, have them shaken around, [exposed to] seasickness, everything else that soldiers are prone to," Milton says.

"Then, the idea was for these ships and tank landing craft involved in this operation, to bring them up toward Slapton Sands where there was going to be shellfire and gunfire so the men would land in real battlefield conditions."

But to ensure the safety of their men and the effectiveness of the whole exercise, Allied Command had to keep the operation a secret — even from their own men.

"They told us nothing. They told us absolutely nothing," 91-year-old Paul Gerolstein tells NPR's Raz. "We didn't know anything."

Gerolstein was a gunner's mate, second class, on a tank landing ship called LST 515. His ship and 299 others were sent into the English Channel, and just after midnight on April 28, 1944, they started their approach toward the British shore.

An Unwelcome Surprise

But the lack of Germans on the shore belied a German presence on the water.

"A German patrol fleet is out in the English Channel," Milton explains. "And quite by chance, it picks up on its radar this enormous flotilla of vessels, and dramatically and suddenly launches attacks on some of the easy pickings of the flotilla."

Near the edge of the flotilla was LST 515, with Gerolstein on board.

"A flare broke over our head, over our ship," Gerolstein remembers. "I said, 'Oh my god, we're gonna get it.' And apparently we didn't. It must have gone under us, see, because [the] LST was a flat-bottom boat. I looked to the stern and saw LST 531 or 532 get torpedoed."

The damage was significant.

"The torpedoes tear into these vessels and literally blow them apart," Milton says. "They all catch fire and there's complete carnage, pandemonium. Men on fire, tanks on fire, the ships on fire. And of course, the ships starting to sink."

Allied commanders, monitoring from London, ordered all the boats to scatter immediately, hoping to avoid any more direct hits from the Germans. But the order left hundreds of men floating in the icy waters.

Gerolstein's commanding officer refused the order and turned his boat back, directing his men to rescue their compatriots still in the water.

"We put cargo nets over the side," Gerolstein recalled. "I went down the cargo net to the last hole. I put my leg through one [hole] and my arm through another one. And as [the men in the water] came by, we'd grab them and pull them onto the net, and they could work their way up."

All told, Gerolstein and the rest of the LST 515 crew managed to save 70 or 80 lives. Later, he recalled seeing the scene clearly for the first time as the sun rose over the water.

"When we got back and then the light broke, you could walk across the dead bodies in the water," he said. "There was over 700 of them killed."

A Second Disaster

Yet the carnage wasn't over. Many of the ships continued on toward the beach at Slapton Sands. Eisenhower had ordered live fire to be used in the rehearsal, because he had wanted to simulate real-world conditions.

"Now, the idea was that the shelling would stop very, very shortly before the American soldiers came onshore, so that the wreckage of war would still be around," Milton said. "The smells of war, the sounds, the shell-blasted beach would be there. But there was a terrible mixup of timings, which meant that as the American soldiers came onto the shore, the British were still shelling the beach. [This] meant the Americans came under devastating friendly fire from the British."

Within minutes, 300 more American troops were dead. Gerolstein helped ferry some of the wounded to the hospital.

"The orders were, in the hospital, you will not ask these men anything," he says. "You will not ask them anything, you will just take care of them."

When the whole affair was over, close to a thousand American troops were dead.

"It's a staggering figure," Milton says. "All the more staggering when you realize that more people were killed in the rehearsal for the landing at Utah Beach than were killed in the actual landing at Utah Beach." Utah Beach was one of the beaches in Normandy that Allied troops charged on D-Day.

The Lessons of Operation Tiger

For nearly 40 years, well after the end of the war, Operation Tiger remained a secret.

"Allied Command did not want the bulk of the troops about to risk their lives going over to Normandy knowing that this disaster had unfolded in the west country of England," Milton explains.

Operation Tiger did have its benefits, however, Milton says. The Allied commanders ordered better life preservers, for one, and made sure each soldier was properly trained on its use. For another, a system was put in place to collect soldiers who were left stranded out in the water.

But perhaps the most important change was to fix the broken system of communication, Milton says.

"All the different command structures and all the different ships involved in the D-Day landings, all these radio frequencies were standardized so that this miscommunication could never happen on the big day itself."

Five weeks after Operation Tiger, hundreds of thousands of Allied troops unloaded onto the beaches of Normandy, a decisive victory that was to be the beginning of the end of World War II.

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