The saga of late 1936, when Edward VIII became the only British king to voluntarily abdicate the throne in order to marry his mistress, Wallis Simpson, has become an international legend. The sentimental ascribe his motivations to love; the cynical, to madness or sexual obsession.

It’s also a historical event that most everyone already knows about. Numerous leading historians, including Anne Sebba, Philip Ziegler, and Michael Bloch, have written biographies dealing either with Simpson or the short-lived reign of Edward VIII, and they have offered insights of startling originality into the circumstances and motivations of the abdication. But no single volume deals authoritatively with the crisis. A new approach to the abdication therefore seemed both wise and timely, even before the departure of both the Duke and Duchess of Sussex from their royal responsibilities.

The sentimental ascribe his motivations to love; the cynical, to madness or sexual obsession.

My research first led me to Balliol College, Oxford. Here, I found an intriguing and hitherto unpublished document. Entitled “He Was My King,” it was the firsthand account of George McMahon, a failed journalist and drifter, who claimed to be an M.I.5 informant recruited by the Italian Embassy with the intent of mounting an assassination attempt on Edward VIII in the summer of 1936. It was lengthy, detailed, and apparently the work of a fantasist.

But there had been an incident on July 16, 1936, in which Edward was troubled by an attention-seeker throwing a gun under his horse. Historians dismissed it as a minor detail, barely worthy of further consideration. Had they missed something?

George McMahon claimed to be an M.I.5 informant recruited by the Italian Embassy with the intent of mounting an assassination attempt on Edward VIII.

Previously suppressed M.I.5 files about the incident confirmed that a great deal of what was in “He Was My King” was, surprisingly, verifiable. McMahon had been a paid M.I.5 informant throughout late 1935 and early 1936. He supplied the organization with details about the workings of the Italian Embassy, and his handlers believed that at least some of the information he was feeding them was “undeniably accurate.”

Therefore, when he began to inform his M.I.5 handlers of an assassination plot, his intelligence should have been taken seriously. It was not, and so McMahon’s attempt went ahead. He was later imprisoned for 12 months, and the only offense for which he was convicted was “possession of a firearm with the intention to endanger life.”

The available documents cumulatively suggested that there had been a cover-up, either because M.I.5 were embarrassed by their previous association with such an unreliable figure or, more tantalizingly, because they had allowed the assassination attempt to take place and wished to conceal their involvement.

My conception of a book about the abdication now shifted inexorably from historical chronicle to suspense thriller.

Tea but No Sympathy

Then there was the question of the King’s character. I consulted all the major British archives—including the Bodleian Library, in Oxford; the Parliamentary Archives, in London; and the Churchill Archives, in Cambridge—exploring the letters and journals of those involved in the crisis. All were universally damning about Edward VIII. But I had yet to find out what those closest to the King really thought of him.

I was granted access to the Round Tower at Windsor Castle, where the Royal Archives held some extraordinary material. There were memoranda discussing Edward’s perceived Nazi sympathies and the effect that they would have on European relations, and candid letters from the future George VI to his mother, Queen Mary, discussing his concerns for the country should he become king. Most useful of all was a private memoir of Alec Hardinge, Edward’s private secretary. Over the course of their association, Hardinge came to believe that the King was dangerously unstable; his revelatory memoir, and other documents, formed the basis of my book.

I concluded that Simpson was a woman “more sinned against than sinning,” which was borne out by the new evidence I discovered. And what I uncovered strongly suggested that Edward’s superficial charm could not conceal the deep, even dangerous, vacancy that lay at the very heart of his being.

In an age where, once again, the world is fascinated by the exploits of a member of the British royal family and his charismatic American wife, it was an education to examine the events of 80-odd years ago and see how little human nature has changed. Yet all the declassified documents and secret letters in existence will never wholly explain what really motivated Edward to abandon his throne for his lover, scandalizing a nation and fascinating the world in the process—a fascination that endures today.

Alexander Larman’s The Crown in Crisis: Countdown to Abdication is out now from St. Martin’s