Prologue ''Thou shalt not kill; but needst not strive Officiously to keep alive.'' At 12.30pm on 16 July 1936, as the massed bands of the Brigade of Guards had passed and King Edward VIII was following on horseback, a limping man in a shabby brown three-piece suit pressed his way through the crowd lining the pavement along Constitution Hill and drew a loaded revolver from his pocket. Edward was a few yards and seconds from death. In a flurry of panic and confusion, the gun left the man''s hand, skidded across the road, and came to rest between the hind legs of the King''s horse. The Annual Register for 1936 recorded that this ''alarming incident created a momentary panic throughout the country.'' The man with the gun was George Andrew Campbell McMahon, a 32-year-old Irishman. At his Old Bailey trial in September he was dismissed by the Attorney General, Sir Donald Somervell, as an attention-seeking eccentric acting in pursuit of a petty grievance against the police.

Pronounced guilty by the jury of unlawfully and wilfully producing a pistol with intent to alarm the King, McMahon was sentenced to twelve months in prison with hard labour. So far, so simple. But MI5, the British security service, had been aware since April 1936 that there was a plot to assassinate Edward: McMahon himself had told them. He informed an MI5 officer with whom he was in regular contact that the attack was planned for June or July and that he was involved. He had even shown the agent the revolver he said he always carried. Interviewed by Special Branch, McMahon told the same story. When McMahon met his MI5 handler on 13 July, he named the day and place the attempt would be made. MI5''s chief, Sir Vernon Kell, discussed the threat with the Metropolitan Police Assistant Commissioner, Norman Kendal.

Despite knowing what was coming - where, when, how - MI5 and Special Branch did nothing: they neither placed McMahon under close observation nor tailed him as he set out from home on 16 July carrying a .36 calibre revolver loaded with four bullets, with more ammunition in his pocket. McMahon had the nerve to ask a mounted policeman to shift out of his line of vision and took the gun from his pocket, yards from the King. Why were MI5 and Special Branch paralysed? Crass bungling, or was something more sinister involved? Were they intentionally standing aside, allowing an attempt on the King''s life to proceed? Edward had no doubt that the threat to his life had been real, masking his concern with an attempt at a joke. His courage in the circumstances was admirable. ''We have to thank the Almighty for two things,'' he later wrote to the general accompanying him on horseback from a ceremony in Hyde Park. ''Firstly, that it did not rain, and secondly that the man in the brown suit''s gun did not go off!'' The King''s mistress and future wife Wallis Simpson wrote after the incident, ''The shot at HM and the upset summer plans have all been very disturbing. No place seems very safe for kIngs.

'' Assassination was in the European air. On 6 May 1932, Paul Gorguloff, a Russian refugee from the 1917 revolution, fired three pistol shots at the French President, Paul Doumer, at the opening of a Paris book fair. Two bullets hit the 75-year-old Doumer, one in the right armpit, the other his head. He died in a hospital bed the following day. Gorguloff, a convicted abortionist, had been angered by what he saw as the French government''s weak attitude towards communists. In court, Gorguloff''s lawyer claimed his client was insane, a plea the jury rejected. Gorguloff was condemned to death and despatched by guillotine on 14 September. Two years later, on 25 July 1934, a squad of Austrian Nazis gunned down Chancellor Engelbert Dolfuss in a failed coup attempt.

The plotters were tried and hanged. Not long after, on 9 October, King Alexander I of Yugoslavia was shot, along with his chauffeur and the French Foreign Minister, Louis Barthou, while driving through the streets of Marseilles at the opening of a state visit. Both the king and the minister died. The assassin, Vlado Chernozemski, a Bulgarian separatist, was slashed by a cavalry sabre, trampled by the crowd and shot by a police officer, dying the same evening. In 1929 the French writer André Breton proposed in his Second Manifesto of Surrealism: ''The simplest Surrealist act consists of dashing down into the street, pistols in hand, and firing blindly, as fast as you can pull the trigger, into the crowd.'' Breton intended no such thing and this display of intellectual terrorism was merely an attempt to shock the bourgeoisie. By coincidence, an International Surrealist Exhibition opened in London in the summer of 1936, with Breton - the ''Pope of Surrealism'' - a dominating presence. The exhibition catalogue claimed for the movement: ''It is defiant - the desperate act of men too profoundly convinced of the rottenness of our civilisation to want to save a shred of its respectability.

'' A few weeks later, McMahon stood on Constitution Hill brandishing a loaded revolver. Cold and efficient, the weapon - even in the hands of an inexperienced gunman - was capable of unleashing five bullets in two seconds. His exploit appeared as senseless as Breton''s projected acte gratuit, an empty gesture. When he came to trial, the prosecution seemed determined to convince an Old Bailey jury that this was indeed the case, minimising the deadly implications of McMahon''s action. The essential difference was that he carried a real weapon, with real bullets, in the presence of a living King. Was McMahon acting out his own surreal fantasy, or had MI5 and Special Branch allowed or perhaps even colluded in something far graver and far more disturbing? There were many in the political establishment and in the royal family itself who had long believed Edward was not fit to be King, even that he represented a danger to national security. King Edward VIII did not die in the way some may have wished in the summer of 1936. The problem of removing an unsuitable monarch found its resolution a few months later with Edward''s self-inflicted social death, his act of abdication.

He and Wallis Simpson - the twice wed and twice divorced ''woman I love'' - went into exile, married, and measured out the decades that followed in an idle faux-aristocratic charade as ''the Duke and Duchess of Windsor''. His conversation often began, ''When I was King.'' The couple''s associates were seedy, moneyed Americans, minor European nobility of dubious provenance, and - with an all too apt irony - their near neighbours the fascist Mosleys, Oswald and Diana. The Duke died in 1972, aged 77, followed fourteen years later by his widow. But Edward had faced the possibility of his actual demise, his physical death, much earlier, while the forces of law and order stood by with arms folded.