Two days before thousands of Allied troops stormed the French port of Dieppe on August 19, 1942, the Daily Telegraph crossword offered the clue French port, six letters, and MI5 launched an investigation into the compiler. It was a fluke, but it captures the chaos surrounding Operation Jubilee, a raid that ended with a 68 percent casualty rate among the mostly Canadian assault force. Drawing on declassified military records and recent research on covert commando units, this episode asks whether Dieppe was a hard-learned lesson that made D-Day possible or a reckless slaughter driven by ego and faulty intelligence.

The episode lays out the competing pressures that produced the raid, from Stalin’s demands for a second front and restless Canadian troops who booed their own prime minister to an RAF Fighter Command hunting for a decisive air battle, and the role of Lord Louis Mountbatten in reviving a cancelled plan. It then examines the intelligence failures, including a German division that had left for the Eastern Front a year earlier, beach assessments based on holiday snapshots, and fortified cliff tunnels no one knew about, before following the assault itself: Churchill tanks chewed apart by loose shingle, a smoke screen that blinded commanders into feeding reserves into the killing ground, and commando successes on the flanks.

• Why the heavy bomber strike and battleship support were cancelled before the landing
• The chance collision with a German coastal convoy that destroyed the element of surprise
• Radar specialist Jack Nissenthall, his cyanide pill, and the bodyguards ordered to shoot him if captured
• David O’Keefe’s theory that Ian Fleming’s unit was sent to seize a four-rotor Enigma machine
• Mulberry Harbours, the Atlantic Wall, and Admiral Ramsay’s ignored warning about training the enemy