The D-Day landings might not have succeeded without the espionage affair related by bestseller Ben Macintyre in Double Cross: The True Story of the D-Day Spies.

This isn’t your first book about spies. What first attracted you to the world of espionage?

Around 1997, MI5, Britain’s internal security service, began to release its classified files. They were so extraordinary and rich in detail, I became completely hooked. Ever since then, MI5 has released more and more material. My books would be impossible to write if they hadn’t done so. The material is unlike any other sort of official material because it’s written by and for people who never expected it to be released. It’s honest in a way that most official files are not.

As much as the success of Double Cross (to deceive Germany about where the D-Day landings would occur) hinged on the five double agents, it also depended on the skills of their MI5 handlers.

These agents were constantly trying to manipulate their handlers on both sides, because their relationships with their German spymasters were just as important as those with their British ones. In a way, the success of the operation hinged on the nature of their personal relationships. What I love about these stories is that although they are war stories, they’re not stories of guns and bravery as we imagine it. They’re about psychology and the very complicated relations between individuals. We’ve always tended to look at the Second World War through a black-and-white prism: you’re either good or bad. People are either collaborators or they’re resistance fighters. This book looks at a shadowy world that involves moral compromises and people, even some disreputable people, who found it within themselves to do something incredible.

Would Double Cross and the related Operation Fortitude have been possible without British code breakers having decrypted the Germans’ Enigma code?

I think it wouldn’t have been possible on the scale that they attempted it.... That said, there were other deception schemes that didn’t rely on intercept material. But it did allow MI5 to have a real-time check on everything, of which the enemy was completely unaware. So that was a critical advantage.

Agent Garbo’s stable of 27 agents—all of them imagined—working for him was a highlight of the book.

It’s irresistible, isn’t it? He was quite mad, a man of absolutely wild imagination. The one thing that I particularly love is the Welsh Aryan Brotherhood. The German obsession with the idea that Wales was filled with would-be Nazis who were ready to rise up was completely fantastic and no such people existed. And yet Garbo managed to invent not one but six of them to pass off against the Germans. He really was behaving like a novelist. He was imagining his characters and filling out his world like a fiction writer.