For Joe Vaghi and tens of thousands of other Americans stationed in England in the spring of 1944, France was the Far Shore, the place where they would finally meet the Nazis in a fight to the death. To prepare, they trained along a stretch of English coast that had been cleared of civilians. It was called Slapton Sands, a tranquil beach that was chosen for its geographic similarity to the coast of Normandy. Their most realistic training was Exercise Tiger, a live-ammunition D-Day rehearsal that involved some 300 ships and 30,000 men in April 1944, six weeks before the invasion.

Gen. Dwight D. Eisenhower, supreme commander of Operation Overlord (Allied code for the invasion itself), was aboard an observers' ship on April 27, when Exercise Tiger went terribly wrong—failed air cover, late landing craft, confusion on the beachhead. Amphibious tanks, heading to shore, misaimed their guns and wounded soldiers on the beach. At least one of the tanks sank in choppy seas while its frantic crew managed to escape. Furious, Eisenhower returned to his headquarters, deeply worried about what the exercise augured for D-Day.

Worse was yet to come. Around 2 a.m. on April 28, nine German Schnellboote—fast, elusive torpedo boats—pounced on a line of eight U.S. tank-landing ships, or LSTs. They were churning down the English coast toward Slapton Sands, fully loaded with vehicles and men who were to land in the next phase of Exercise Tiger. German torpedoes struck three LSTs at the end of the line. LST-531 capsized and sank within minutes, taking hundreds of men down with her. The torpedo that hit LST- 289 crumpled her stern, but she stayed afloat and made it to port. Gasoline aboard LST-507 exploded and set the ship afire.

Eugene Eckstam, a medical officer on the 507, raced for the tank deck, which was filled with men and vehicles. "I saw only fire—a huge, roaring blast furnace," he later wrote. "Trucks were burning; gasoline was burning; and small-arms ammunition was exploding. Worst of all were the agonizing screams for help from the men trapped inside. But I knew there was no way I—or anyone else—could help them. I knew also that smoke inhalation would soon end their misery, so I closed the hatches into the tank deck and dogged them tightly shut."

Senior officers ashore, quickly assessing the damage, ordered the five surviving LSTs to continue steaming toward Dartmouth, their destination. Capt. John Doyle, commanding officer of LST-515, the lead ship, disobeyed the order. He turned back to look for survivors. "We started looking for the ones who were still alive," Brent Wahlberg, 515 gunnery officer, remembers. "We found 132 survivors." Many of the dead, they noticed, were floating head down, feet up, with their life belts inflated. No one had told them that the life belts were to be worn under the armpits, not around the waist. That lesson from Exercise Tiger would be taught to invasion troops, saving countless lives.

In the meantime, Eisenhower had an intelligence nightmare on his hands. To keep the disaster a secret, he ordered extraordinary measures and kept the lid so tight that the details remained unknown for many years. He did not want Germany or Allied troops to know about the huge loss of life—749 sailors and soldiers by final count. Of even greater concern was the possibility that the Schnellboote might have picked up survivors who carried the most closely guarded secret of the war in their pockets—the location of the D-Day landing.

One simple word, BIGOT, is stamped in big letters across the Operation Neptune Initial Joint Plan of February 12, 1944, and from then until June 6, that stamp appeared on all supremely secret pieces of paper handled by D-Day planners. If any of those papers or maps had fallen into enemy hands, the invasion would have failed or been scuttled—a distinct possibility in the anxious days after Exercise Tiger.