Between 1939 and 1945, Germany's Abwehr intelligence service sent approximately 115 agents to the United Kingdom. MI5 identified and captured every single one. Not some of them. Not most of them. Every single one was either captured, surrendered voluntarily, or was already a double agent before arriving.

This gave Britain an extraordinary advantage: complete control over what Germany believed about Allied military capabilities, troop movements, and intentions. The operation was managed by the Twenty Committee — named with a pun, as "XX" represents both the Roman numeral for 20 and a "double cross."

The Double Agents

Captured spies were given a choice: work for Britain or face execution. Most chose to cooperate. Each was assigned a case officer and began sending carefully scripted messages back to their German handlers. The information was a precise mixture of true but harmless details (to maintain credibility) and false intelligence (to achieve strategic deception).

The most famous double agents included:

TATE (Wulf Schmidt) — A Danish-born agent who parachuted into England in 1940 and was turned within hours. He transmitted for four years, becoming so trusted by the Abwehr that they awarded him the Iron Cross — which MI5 officers accepted on his behalf with considerable amusement.

TREASURE (Nathalie Sergueiew) — A French-Russian woman whose beloved dog Babs was quarantined upon her arrival in Britain. When the dog died, she was so distraught she threatened to reveal the entire operation to the Germans. Her MI5 handler talked her down.

BRUTUS (Roman Czerniawski) — A Polish officer who had actually run a real intelligence network in France before being captured. He was turned by MI5 and became crucial to the D-Day deception.

Legacy

The Double Cross system was perhaps the most successful intelligence operation in history. It kept the D-Day deception intact, misdirected V-1 and V-2 rocket targeting away from central London (saving thousands of lives), and ensured that Germany's entire intelligence picture of Britain was controlled by MI5.

The system's existence was not revealed until 1972, when Sir John Masterman, the committee's chairman, published his memoir over the objections of MI5.