The USA has always been obsessed with traitors, real and imagined, from John Wilkes Booth to the Rosenbergs and beyond. Of all of them, it is the War of Independence officer Benedict Arnold that looms largest, perhaps because the so-called ‘victims’ of his treachery were never able to catch him. The volume of abuse expended on him is extraordinary. Benjamin Franklin contrasted him unfavourably with the betrayer of Jesus. ‘Judas sold only one man, Arnold three millions,’ he remarked.

In 1780, Arnold, then commandant of West Point fortress on the Hudson River and a favourite of George Washington, conceived a plot to betray the fort and its three thousand-strong garrison to the British. West Point was considered impregnable to normal military methods (as Vicksburg would be viewed during the American Civil War, except that the British did not have a military genius like Ulysses S Grant to compass its fall). Using Major John André as ‘cutout’, or intermediary, Arnold negotiated with General Henry Clinton to deliver fortress, garrison and all artillery and magazines to the British, who hoped that this would alter the course of the entire bloody war. After much haggling, Clinton agreed to pay £20,000 to Arnold if the plan was successful, but guaranteed him a ‘floor’ of £6,000 if the plot miscarried. André saw Arnold as a ‘Monck’ figure: General Monck, a former Cromwellian commander, was the key individual in effecting Charles II’s restoration in 1660. As seems to happen in the majority of conspiracies, a series of contingent aleatory events meant the plot was discovered. Arnold escaped by the proverbial skin of his teeth, just minutes ahead of his pursuers. The luckless André, who barely disguised his uniform with civilian clothes for his rendezvous with Arnold, was caught, tried as a spy and executed. Washington offered to trade André for Arnold, but Clinton refused.

What made the case so sensational was that Arnold at the time was the American rebels’ premier military hero. In 1775, after an undistinguished early life, Arnold shot to the top at the age of thirty-four. He won fame in the closing months of that year at a siege