Bletchley Park and D-Day

Image of Bletchley Park and D-Day
Release Date: 
May 28, 2019
Yale University Press
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“Bletchley Park and the Ultra codebreakers have been credited by some historians as potentially shortening the war by a year or two . . .”

One of the more recently covered topics in World War II history has been the intelligence war, particularly the Allied effort to conduct intelligence and counter-espionage operations to support the D-Day landings. One of the subjects of recent notoriety is the Allied code-breaking program known as Ultra. Highlighted in the recent movie The Imitation Game, the effort by mathematicians and cryptologists at England’s Bletchley Park to crack the German Enigma cipher machine have passed into something of a legend in the intelligence world.

But as this new book by the research historian of Bletchley Park lays out, the decoding efforts were just one part of a massive Allied intelligence endeavor to provide a systematic intelligence source for the planners of the D-Day invasion. The value of the Ultra collection and analysis was not simply the ability to decrypt German messages, but the concentrated effort to turn those intercepts into useful intelligence that could be used by Allied planners.

As the author notes, the British Government Code & Cipher School, GC&CS for short, eventually set up its own complete intelligence service, providing not only decryption, but the entire process of signals intelligence against Germany’s supposedly unbreakable codes.

From the time a signal was picked up, the well-organized bureaucracy at Bletchley Park handled the entire process of creating intelligence—collecting and decrypting signals; translating and collating of the various messages; cross-referencing and indexing the information gleaned from the messages; and finally creating their own reports for distribution to Allied political and military leaders via their own network of intelligence liaison officers.

Because of the avalanche of daily traffic and the need to crack many Enigma settings on a daily basis, the author uses a lot of statistics to show that a huge factor in Bletchley’s success was in determining what German signals to concentrate their limited decryption personnel and machines on attacking.

The ultimate strength of Allied intelligence efforts was their ability to combine the resources of both American and British code-breakers by 1944 to produce an amazing intelligence organization that provided a clear strategic advantage to the Allies.

An interesting side note is that American efforts to crack Japanese codes not only aided American war efforts in the Pacific theater, they provided a bounty in intelligence on German war efforts as well. Since Japan retained a full diplomatic and military mission in Germany throughout the war, the Japanese ambassador and military attaché often sent long and detailed reports back to Tokyo that were intercepted and decrypted by U.S. Navy codebreakers and sent to the Allied intelligence staff in London. These reports contained detailed descriptions of Germany’s invasion preparations and were a vital supplement to the Ultra effort.

The merging of Ultra reports, air reconnaissance photos, reports from French resistance agents, and the new art of radio traffic analysis allowed the Allies to have a nearly complete picture of the German Army’s order-of-battle in France. More importantly, Ultra allowed the Allies a better understanding of the German’s plans and intentions to meet an Allied landing on the shores of France, all of which was taken into consideration by the staff planning the D-Day landings.

The author also handles the controversies concerning Ultra reporting, particularly the failure to report on the movement of three German divisions, the 352nd Infantry, 21st Panzer, and 91st Airlanding Division, just prior to the invasion.  Although the Allies discovered about the movement of the 91st in time to adjust the drop zones of the two American airborne divisions, the 352nd moved into position along a wide swath of the coast, including along Omaha and Gold Beaches undetected by Allied intelligence. 

Historians have long considered this a major intelligence failure and the cause of the significant casualties of American troops on Omaha Beach, but as the author notes, the division was widely scattered along the shore and only about two or three companies of infantry were actually assigned to the beach defenses. As well, the 21st Panzer Division was much closer to the beach than the Allies thought, and the division ended up along the main British march route toward the city of Caen, significantly slowing the advance and upsetting the original British plan to capture the city by the end of D-Day.

In spite of these shortcomings, the intelligence picture of the German order-of-battle and intentions of the German commanders were remarkably accurate, as post war historical analysis would confirm. Moreover, once the Allies were firmly ashore, Bletchley Park continued to provide useful intelligence about German troop movements and plans, particularly the German counterattack toward Avranches that attempted to stop the American breakout of Operation Cobra. It was only when the Allies began their breakout and pursuit of the German forces across France that Ultra could no longer keep up with the rapid tactical pace of the battle.

As the author notes in the conclusion, Bletchley Park and the Ultra codebreakers have been credited by some historians as potentially shortening the war by a year or two. Certainly the contributions of the codebreakers and intelligence analysts gave the Allies a far clearer picture of German capabilities and intentions than their counterparts had of the Allies. The remarkably well integrated effort by myriad specialists truly showed how intelligence can provide a clear strategic advantage, in this case making the highly risky D-Day operation a little more possible.