It was one of the Nazis’ most audacious plans. In June 1942, the Third Reich dispatched eight saboteurs to the United States.

Armed with fuses, pre-made bombs, TNT and $82,000 (over $1m in today’s money), their goal was to blow up key military and transportation installations, power plants, aluminium factories, and canal locks and bridges, including New York’s famous Hell Gate Bridge.

They were also to spread terror by attacking Jewish-owned department stores.

The men — German-born but all of whom had spent some time living in America before returning to the fatherland — arrived by u-boat in early June. The first team came ashore at Amagansett, Long Island; the second followed five days later, landing near Jacksonville in Florida. They were to meet in Cincinnati on July 4 to begin their mission.

But, within two months of their arrival, six of the eight men had been sent to the electric chair, while two of their supposed comrades were beginning long prison sentences.

Operation Pastorius, named after the founder of the first German settlement in colonial America, is a curious tale — of incompetence, betrayal, high-level cover-ups and the perversion of US justice — with an eerie postscript.

Although hatched by Walter Kappe, a senior Abwehr officer who had spent the 1930s living in America, the would-be saboteurs were hardly the Third Reich’s finest. Gary Cohen, who has studied hours of FBI interviews and 3,000 pages of court transcripts, labelled them “the keystone commandos”.

George Dasch, Kappe’s first recruit and the man who would lead the group, had spent 20 years in the US, mainly working as a waiter, before returning to Germany in 1941. While speaking near-perfect American English, he turned out to have little sympathy for the Nazis and had been baulked when Kappe first told him the US was the target. Another recruit, Ernest Burger, was a former stormtrooper and one of the Nazi party’s early members — but a university paper critical of the Gestapo had landed him in a concentration camp, from which he had been released just months before being recruited by Kappe. Still another, Heinrich Heinck, spoke only a smattering of English, delivered in a thick German accent.

At the same time, their training in explosives at a farm outside Berlin was thorough and their false identities carefully constructed. An element of fear was drilled into them: if any showed signs of weakness once they had arrived in the US, the others were to kill him immediately. The FBI, they were also warned, had been heavily infiltrated by German spies; no one should think of attempting to defect.

However, Kappe’s plans began to go awry as soon as Dasch’s team set foot on American soil. Dressed in German uniforms — so they would be treated as prisoners of war if they were captured when landing — the men were spotted by a young, unarmed coast guard. Dasch attempted to bribe him but a key witness, who would soon raise the alarm and thus swiftly facilitate the discovery of a cache of buried uniforms and weapons, had been allowed to slip away.

A nationwide manhunt was soon under way but the saboteurs were delivered into the FBI’s hands by two of their own. Arriving in New York, Burger — who had already left a deliberate trail of clues on the Long Island beach — and Dasch agreed to contact the authorities. Dasch took the lead and, with his assistance and that of Burger, by June 27, all eight men were in custody.

Rushing to claim the glory, FBI Director J Edgar Hoover announced the men’s capture. “The country went wild,” wrote Attorney General Francis Biddle in his memoirs, while noting that Hoover’s failure to mention the role of the coast guard and Dasch left the public believing that the plot had probably been smashed by “a particularly brilliant FBI agent”

It was not just the public who were deliberately misled: confidential memos from Hoover to President Roosevelt failed to mention that Dasch had turned himself in.

Another betrayal was soon afoot. The FBI convinced Dasch that he should plead guilty, assuring him that he would be pardoned by the President and released within three to six months. Roosevelt, though, had already decided that all the men must die and the civilian courts — which, War Department lawyers concluded, might only convict them of conspiracy to commit a federal crime and jail them for two years — should be circumvented.

Using a Civil War-era precedent, a presidential proclamation announced the formation of a military tribunal (as George W Bush would later attempt to instigate for Guantanamo detainees). Held in secret, it found all eight guilty and recommended the death penalty, while also suggesting that Dasch and Burger be spared. With the Supreme Court refusing to intervene, the men’s fate rested with Roosevelt. He ordered the immediate execution of all the men bar Dasch and Burger, who received sentences of 30 years and life respectively.

Dasch would never receive the promised presidential pardon. He and Burger were released after six years and deported to West Germany. Treated as a traitor at home, Dasch was refused all requests for a visa to return to America at Hoover’s instigation.

But the six men who were executed 75 years ago this week, and buried in unmarked graves in a wooded area of south-west Washington, were not entirely forgotten. In 2006, Jim Rosenstock of the National Park Service was alerted by workmen to the discovery of a 200-pound granite block. Regularly cleaned, and with candles around it, it was engraved with the words: “In memory of agents of the German Abwehr, executed 8 August 1942”.

Below their six names, was a final, chilling inscription: “Donated by the NSWPP”. The National Socialist White People’s party was the name with which, shortly before his assassination in August 1967, George Lincoln Rockwell rechristened the American Nazi party.