One April morning in 1943, a sardine fisherman spotted the corpse of a British soldier floating in the sea off the coast of Spain, setting in train a course of events that would change the course of World War Two.

Operation Mincemeat was the most successful wartime deception ever attempted, and certainly the strangest. It hoodwinked Nazi espionage chiefs, sent German troops racing in the wrong direction, and saved thousands of lives by deploying a secret agent who was different, in one crucial respect, from any spy before or since: he was dead. His mission: to convince the Germans that instead of attacking Sicily, the Allied armies would invade Greece.

The brainchild of an eccentric RAF officer and a brilliant Jewish barrister, this great deception involved an extraordinary cast of characters including Ian Fleming, who would go on to write the James Bond stories; a famous forensic pathologist; a beautiful secret service secretary; a submarine captain; three novelists; an irascible admiral who loved fly-fishing; and a dead, Welsh tramp. Using fraud, imagination and seduction, Winston Churchill's team of spies spun a web of deceit so elaborate and so convincing that they began to believe it themselves. From a windowless basement beneath Whitehall, the hoax travelled from London to Scotland to Spain to Germany and ended up on Hitler's desk.

The two masterminds of the operation were Charles Cholmondeley and Ewen Montagu, two men with what Churchill called "corkscrew minds". First, they obtained a dead body from a London coroner named Sir Bentley Purchase. Then they set about creating an entirely new personality for the dead man, whom they designated William Martin, a major in the Royal marines. Using false identity cards, faked personal letters, receipts, bills, photographs and other "wallet litter", they gave the dead man a father, a fiancee and a backstory. But most crucially they chained to his wrist a briefcase containing official-looking documents, letters written from one general to another, which clearly indicated that the Allied armies massed in North Africa were aiming for Greece. These, too, were false.

Montagu and Cholmondeley had a specific target in mind for their elaborate deception: Adolf Clauss, a German spy operating in the port of Huelva on Spain's south coast. He was known to be efficient, ruthless and extremely gullible.

In April 1943, the body was dropped into the sea from a Royal Navy submarine and then floated towards the coast of Spain.

Operation Mincemeat fooled Hitler completely: German troops were deployed to the wrong place; thousands of British, Canadian and American lives were saved; Mussolini was deposed; and the course of World War Two was changed.

By Ben Macintyre, author of Agent Zigzag and Operation Mincemeat