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Bletchley Park and D-Day

The Untold Story of How the Battle for Normandy Was Won


David Kenyon


New Haven: Yale University Press, 2019

. Hardcover. xxx+295 p. ISBN 978-0300243574. $28/£20


Reviewed by Mary Kathryn Barbier

Mississippi State University




For decades, popular culture has shaped the general public’s understanding of intelligence history. Movies such as Alfred Hitchcock’s ‘Secret Agent’ (1936), ‘Sabotage’ (1936), and ‘Notorious’ (1946); Dr. No (1962) and other James Bond movies; or Cold War spy films like ‘The Good Shepherd’ (2006), Brad Pitt’s ‘Spy Game’ (2001), Pierce Brosnan’s ‘The Tailor of Panama’ (2001), and Angelina Jolie’s ‘Salt’ (2010), portrayed the exciting world of spies and perhaps colored perceptions about what intelligence history is. Equally, spy novels, to some extent, can shape the way historians write about those involved in the collection of intelligence, particularly since authors like Ian Fleming, John LeCarré, and Graham Greene, who worked with British Intelligence during World War II, brought a unique perspective to their books.

Popular culture has, therefore, suggested that intelligence history is sexy, exciting, and suspenseful, and some, but not all, of it is. Some scholars of intelligence history write about the men and women, who were on the front lines as spies or double agents. A few books immediately come to mind: Robyn Walker’s The Women Who Spied for Britain (2014); Robert Whymant’s Stalin’s Spy : Richard Sorge and the Tokyo Espionage Ring (1996); Sophie De Schaepdrijver’s Gabrielle Petit : The Death and Life of a Female Spy in the First World War (2015); and Ben Macintyre’s The Spy and the Traitor : The Greatest Espionage Story of the Cold War (2018). While the books about these and other spies and double / triple agents are interesting and exciting, they only present part of the story.

Not all spies had ‘James Bond’ type experiences, nor did they necessarily have ‘cool’ gadgets to make their jobs easier. Furthermore, the collection of intelligence is more than a story about the exciting lives of those tasked with acquiring it.  Some historians focus on the less sexy topic of information transmission – communiqués sent by trusted agents in the field or the intercepted messages of enemy agents. Captured encrypted messages presented challenges to those trying to glean important information from them. Consequently, a key component of communication is codebreaking, which is currently a hot topic for historians and enthusiasts. Books about codebreaking frequently focus on the people involved in deciphering; however, the story about those engaged in codebreaking during World War II is a complicated one. Because attention is more on the actors and less on technology – for example, in books like Leo Mark’s Between Silk and Cyanide : A Codebreaker’s War, 1941-1945 (1977); David Kahn’s Seizing the Enigma : The Race to Break the German U-Boat Codes, 1939-1943 (2012); or Lisa Mundy’s Code Girls : The Untold Story of the American Women Code Breakers of World War II (2017) – these books appeal to a broad audience, but the reality is much more complicated. In addition to codebreaking, someone has to understand the value of the intelligence and disseminate it to the people who can use it to achieve a desired end – in the case of World War II, victory on the battlefield.

In his most recent book, Bletchley Park and D-Day : The Untold Story of How the Battle for Normandy Was Won, David Kenyon enhances an understanding about how intelligence, particularly that transmitted by wireless, was acquired, how it was read, and the impact that it had on the Allies’ invasion of, and campaign in, Normandy. As Bletchley Park’s research historian, Kenyon is uniquely placed to dig deeply into the history of the Government Code & Cypher School (GC&CS) at Bletchley Park and its role in providing actionable intelligence to the planners and implementers of the Allies’ military and political strategic plans to emerge from World War II victorious. While acknowledging that other authors, including some who worked at Bletchley Park, have written books about the GC&CS and the work done there, Kenyon explains what he adds to the work of his predecessors, such as Ralph Bennett and Harry Hinsley, and what makes his monograph different from theirs and those of other scholars. He argues that he does not just focus on the mundane details and minutiae, but on the important contribution that Bletchley Park made to the Normandy invasion and subsequent campaign. 

Kenyon argues that to understand the importance of Bletchley Park, historians have to examine more than the work of Alan Turing or of the first computers. They have to explore the outcomes. Did the intelligence that Bletchley provided to Allied military commanders contribute to success on the battlefield in Normandy? In order to answer that question, however, one has to understand the process from the interception of communiqués, to codebreaking, to analysis of the information, to dissemination in time to make a difference. Consequently, Kenyon meticulously explains the expansion of the facilities at Bletchley, the development of the technologies – the Bombe and Colossus – that facilitated codebreaking efforts, the growth of GS&CS, and the cryptological process of encoding and decoding. Then he focuses on the huts – 3, 6, and 8 – whose work most directly affected the Normandy invasion. Hut 3 was the Army and Air Force Enigma Reporting Section. Tasked with decryption, the members of Huts 6 focused on army and air force messages while those in 8 housed concentrated on naval communications. From its inception, the network at Bletchley continued to evolve throughout the war. By the eve of the Normandy invasion, it had become a sophisticated, efficient organization with assembly-line outputs.

As Kenyon notes, Bletchley Park did more than decrypt intercepted wireless messages. Traffic analysis (T/A) played ‘a huge part in painting the intelligence picture of the German forces prior to the invasion’ [68]. Relying on multiple sources, not just Enigma messages, Bletchley consistently constructed German orders of battle in the months before the invasion. By analyzing Japanese diplomatic communiqués as well as German and Italian military messages, the GC&CS provided insight into the Germans’ defensive preparations, their assessment of Allied troop strength and invasion preparations, and their projected ability to move additional forces into Normandy once the invasion began. Because the amount of enemy wireless traffic increased, the staff at Bletchley continued to work flat out to deliver up-to-date information to commanders in the field although they did not always receive intelligence about enemy operations before they commenced. When possible, Allied air forces used Bletchley’s information to interdict German panzer divisions to delay their arrival and ability to counterattack.  While they continued to provide information about German orders for the movement of panzer divisions to Normandy as the campaign unfolded, Kenyon argues that ‘Bletchley Park played its biggest part in OVERLORD before any troops landed on the beaches and landing grounds, as a tool for Allied planners and politicians’ [246-247].

While the amount of incoming traffic flowing was potentially overwhelming, the teams at Bletchley made decisions about which messages warranted decoding and what information to include in the daily intelligence summaries earmarked for dissemination to Prime Minister Churchill and military leaders, both British and American. Doing so facilitated the creation of a clear picture of German intentions, planning, and thinking. Abwehr (the German military intelligence service), on the other hand, did not analyze or synthesize the intelligence before passing it up the chain. As Kenyon eloquently states: ‘Unlike Bletchley, which went to great lengths to filter and rationalize its intelligence product, the Abwehr took the opposite approach and bombarded its customers with a deluge of unfiltered material – proof that too much intelligence is as unhelpful as too little’ [244]. Even if the intelligence was actionable, it was frequently lost in the midst of the raw data that the Abwehr provided.

This was also a story about collaboration. Kenyon does a good job of teasing out the relationship that the British and American intelligence communities forged. By the spring of 1944, American cryptanalysts, intelligence officers, and staff were fully integrated into the Bletchley Park organization. They had their own space and bombes, and there was an agreement upon sharing of intelligence information by both parties. Once on the continent, Special Communications Units (SCUs) were embedded in field headquarters and ‘tasked with the reception of ULTRA messages from Bletchley Park’ [xxiv]. While some commanders remained hesitant to avail themselves completely of what the SCU had to offer, others, such as General George S. Patton, welcomed the intelligence that they provided.

What made parts of the book more engaging were the vignettes and the voices of Bletchley actors whom Kenyon included in the book. More of those vignettes and voices would have enhanced the work, and a broader audience would have found the narrative more accessible. In all likelihood, however, doing so would have taken the narrative down a different path from that which the author intended. While some readers might get bogged down in the details that Kenyon included, those interested in the process end (codebreaking, encryption, decryption, and the early Bombe and Colossus computers) will find much to capture their attention.

It is clear from Kenyon’s book that the work at Bletchley Park was much more complicated than previous narratives and first-hand accounts have demonstrated. Both Ralph Bennett and Harry Hinsley worked at Bletchley but were not privy to everything that went on there; therefore, their accounts are incomplete, Kenyon suggests, and colored by their own experiences. As an official historian, however, Hinsley had access to many more details about Bletchley than Bennett did, and as a result he provided a richer history than Bennett’s. Many of these books focused more on the codebreaking than on analyzing the impact that the GC&CS had on the battlefield, i.e., planning and execution of operations, in general. Kenyon took a more nuanced and targeted approach by analyzing the Park’s effect on one major campaign – the Normandy invasion. While some intelligence provided by Bletchley had an impact on tactical operations, its more important contribution was on the big picture or strategic level. As Kenyon aptly argues, ‘The idea that Bletchley Park’s job was only to turn Enigma into ULTRA obscures the true picture of GC&CS as a fully developed, multi-source intelligence agency, drawing on a much wider range of sources and delivering a much wider range of products’ [241-242]. Bletchley Park and D-Day makes an important contribution to the historiography of World War II intelligence history in general and, more specifically, of Bletchley Park and to a broader understanding of the Bletchley players, who, Kenyon notes, have not received the attention that others who played a role in the successful Normandy invasion have.



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