Photo of Agent Garbo courtesy http://media.npr.org.
I just finished reading the book Double Cross: The True Story of the D-Day Spies which explains the role of double agents in the Allied Victory on D-Day. Britain employed several agents to glean information from and feed information to the Germans during the Second World War. The author, Ben Macintyre, maintains that without these agents, the Germans likely would have been better prepared for the D-Day invasion and the result would have been a massacre of the Allies and possibly an Axis victory.
Code-named Brutus, Bronx, Mutt, Freak, Tricycle, Treasure, Tate and Garbo, these agents were not only working for Britain but for Germany as well, hence the term double agent. The most effective of the spies, Garbo, was an anti-Nazi, anti-Communist veteran of the Spanish Civil War named Juan Pujol Garcia. He persuaded a German agent that he had a visa to travel in Britain, thus securing a job as a German spy. However, the visa was forged and Garbo was forced to go to the Madrid Public Library and do research there. He made up three sub-agents and talked about their hilarious adventures in Britain in his reports to Hitler. The Fuhrer bought it hook line and sinker.
By mid-1942, Garcia pulled some strings to get to England where he continued to write reports for German intelligence sent via radio or safe-deposit box. This time, the reports, detailing a network of 24 spies, were written by British intelligence and Garbo was just the messenger. Garbo and the other double agents were able to convince Hitler and the Wehrmacht that the Allies were planning to invade Norway and constantly a good number of troops were dispatched there. More importantly, the spies claimed that the Allies would invade France at the Pas de Calais (which would make for a short English Channel crossing for the British) rather than at Normandy (the actual invasion site). Hitler ordered that a significant number of troops remain at the Pas de Calais thereby limiting the number of soldiers at Normandy.
To accompany these false reports, the Allies provided false planes, false tanks and false generals (an actor played "Monty", one of the main generals). The ruse was sufficiently plausible and the German intelligence bought it. On June 6, 1944, in the wee hours of the morning, the Wehrmacht received word from the double agents that the D-Day attack would be at Normandy afterall -- but it was too late to make any significant changes. When the Allies stormed the beaches at Juno, Omaha, Utah, Sword and Gold, they were met by German resistance. But they also had the element of surprise and were able to take the coast, the country, and eventually the continent.
Ironically, Agent Garbo was rewarded by Hitler with the Iron Cross First Class for his work as a spy. If not for the double agents, World War II could have had a very different outcome.
Photo of D-Day Invasion courtesy www.rjgeib.com.