People believe that with magic, it’s the quickness the hand that deceives the eye. They’re wrong. The eye is drawn to sudden movements. If you don’t want people to notice what you’re doing, do it slowly and calmly while they’re looking somewhere else.

I learned about stage magic while I was researching one of the least understood aspects of World War II: the deception war and the eccentric British soldier behind it, Dudley Clarke.

If you ran into Colonel Clarke at a bar in Cairo in 1942, he’d have bought you a cocktail and told you a funny story. Everyone liked him, but no one seemed to know what he did. Some suspected him of drinking all night and sleeping all morning.

In reality, between 1941 and 1945, Clarke was re-inventing the practice of military deception, taking it to levels not seen before or since. By the end of the war, Clarke had persuaded the Germans of the existence of whole armies, fleets, and bomber wings—tens of thousands of fictional soldiers. These imaginary units were placed in areas where they threatened Adolf Hitler’s defenses, driving him to deploy his own forces, thereby distracting hundreds of thousands of real troops with fake ones. It was a feat so audacious that almost no one, even officials at the top of the U.S. and British armies, grasped what he had achieved.

British troops carrying a dummy Stuart tank, April 1942.

Clarke was a career army officer and a favorite among generals for the way he would “tackle any emergency with calmness, almost with unconcern,” as Field Marshal Archibald Wavell said. He knew how the military worked, what his commanders wanted, and how to get what he needed. Plus, his uncle Sidney Clarke had been one of Britain’s leading amateur magicians and taught a young Clarke the secrets of stage magic.

One of those secrets is that going slowly is better than going quickly. The other is that performing magic tricks right is really tedious.

While writing my book, I learned a two-minute illusion, designed for me by the British magician Shane Miller. It involves four separate sleights of hand, and it took me months of practice before I was ready to perform it. I repeated the same actions again and again, sometimes in front of a mirror or a camera, sometimes as I watched TV or read a book. Even when I thought I had it ready to film, it took me hours of repeated takes to get one that was good enough to use.

Between 1941 and 1945, Clarke was re-inventing the practice of military deception, taking it to levels not seen before or since.

In the words of the magician Teller, “Sometimes, magic is just someone spending more time on something than anyone else might reasonably expect.” That generally means continual practice, but it can also mean going to unreasonable lengths: memorizing the order of an entire deck of cards, for instance, or hiding objects days or weeks before you’re going to need them.

This was precisely how Clarke approached creating his imaginary armies. He had, in his own words, a “capacity for taking infinite pains which lay well-hidden behind an easy-going exterior.” He planted mentions of units for his audience to find months, or even years, before they were needed. When it was time to “deploy” one of those fake troops, their existence had already been a fact for German intelligence officers, so no one thought to question it. Clarke’s team kept detailed records about the activities of these fake forces, ensuring the story was always consistent.

In the run-up to D-Day, 40 Axis divisions were estimated to have been held in place, far from Normandy, due to Allied deceptions. After the war, German generals were baffled that the Allies hadn’t deployed units that they were sure existed.

Putting things in place long before you need them, avoiding attention-grabbing actions, and meticulously planning—that’s magic. The slowness of the hand deceives the eye, and the enemy.

Robert Hutton is a freelance journalist and the former U.K. political correspondent at Bloomberg