Via The Daily Beast:
Read a history of double agents during wartime:
The double agent remains the most prized, the most feared, and the most unreliable weapon in the espionage armory. Sun Tzu, the ancient Chinese military strategist, stressed the supreme importance of having a spy within the enemy’s spy camp; double agents plied their shadowy trade to remarkable effect during the intelligence duels of the First and Second World Wars, and the Cold War. And double agents continue to play a vital part in the modern intelligence battle that underpins the War on Terror.
Continue at The Daily Beast.