Before you begin reading this book, take a look at the map at the very front. It's a map of northern France and southern England. Notice how close the cities of Dover and Calais are; the sea distance is about 21 miles. Meanwhile, continue west to the widest gap between France and England which is about 100 miles. That's the distance between Portsmouth, England and the five Normandy beaches. Those 100 miles were crossed by the American, British, and Canadian forces on June 6, 1944 - D-Day. Why the Allied forces chose to set the invasion on this particular plot of land in France, reachable after an all-night trip from England, is the topic of many other books about WW2. This book, "Double Cross: The True Story of the D-Day Spies" by Ben MacIntyre, is the story of how British intelligence worked to make the Germans think the imminent invasion would occur at Calais, rather than Normandy.
By 1942 the smart money in Germany was on an invasion on the European continent in France or Scandinavia by Allied forces. It was thought to be both inevitable and somewhat imminent. The time factor was based on many things, including build-up of invasion forces, the war effort in other European theater sites, and, of course, the geography of France. Just looking at a map shows the shortest distance was from Dover in Kent to Calais - as I wrote before, about 21 miles. Hitler and the German High Command were expecting the invasion in that area, and had mined the beaches and inner area in preparation for repelling an invasion. There were many troops stationed in the area, too. But, the Germans also mined and prepared the Normandy beaches with the same mines and hill top fortifications, though not as many as in Calais and they also had fewer troops stationed in Normandy.
British intelligence - MI5 (national) and MI6 (international) were cooperating in controlling German spies inside the UK. The "idea" of spies - both homegrown and dropped in - was much worse than the truth of the matter. Britain actually had captured and either turned or executed all the spies who had been dropped in by air or had landed by sea. These German "spies" tended to be a dismal lot of dullards who ineptly gave themselves away by various means to the Brits who would literally stumble upon them. But a few of the spies could be used to broadcast back to Germany what their British controls ordered them to say.
Another group of spies were individuals - in this case, French, Polish, Spanish, and Peruvian(!) - who had come to British embassies in Lisbon and Madrid and offered their services to the Allied war effort. Their reasons ranged from mercenary (nearly everyone's) to patriotic (the Polish man who had set up a spy network in France that had been broken up by the Germans before he volunteered to help the Brits). Most were double crossing the Germans who also handled them. With this patched together network, based in London, Madrid, and Lisbon, British intelligence was able to hone the message their new agents were sending back to Germany. Honing the message to throw the Germans off the true site of the inevitable invasion, from Normandy to Calais and to the timing of the invasion. Using fake letters, some "real" but not important bits of intel, and even the "sighting" of British General Montgomery in Gibraltar, by using an actor to play Montgomery. If "Monty" was in Gibraltar in late May, then how could he possibly be working on the upcoming invasion?
Ben MacIntyre - who also wrote an excellent book a couple of years ago called "Operation Mincemeat" about the attempt to throw German intelligence off the prospective Allied invasion of Sicily - returns here with a sometimes humorous, but always serous, story of the spies, the spy masters, the actors, the military staffs, who together pulled off the successful trick of getting the Germans to consider the Pas de Calais as the ONLY invasion area, even after the main invasion had begun on June 6th! Still expecting the invasion to take place to the east, many German troops were kept back from that area, joining too late their fellow soldiers repelling the Allied forces at Normandy to the west. MacIntyre's book on D-Day and the spies and agents who kept the secret and helped focus German intelligence on other places and other times, is filled with characters usually found only in fiction. I suppose it takes a certain individual with the flair and guts to pull of a double crossing spying job, and the five or so foreign spies who were on Sir John Masterman's "Double Cross" system payroll were an intriguing lot. This book is a must-read for the WW2 buff. (By the way, his writing on pigeons - both German and Allied - being used in spying, is a hoot.)