Access to the decrypted messages of the German Enigma, called ULTRA, was very limited. Everyone working for Bletchley Park (BP) or in contact with BP had to sign a life-long secrecy act with stiff consequences for those who would break the silence.

As a rule: for ULTRA information to be used there had to be a second source to mask the fact that the Enigma had been broken. On top of that it was avoided to have tactical commanders to act upon this ULTRA INTEL because it would risk that the Germans found about it. Also not on every bit of ULTRA was acted, also because it was a lot and not all information was that clear.

We know that Montgomery had access, and all of his Army commanders, as well as their chief Intelligence Staff Officers (ISO). More specifically: did Lieutenant General Sir Brian Horrocks, commander of the XXXth British Corps have access to ULTRA messages in Operation Market Garden?

We know now - since the secrecy on ULTRA has lifted - that there was overwhelming ULTRA INTEL that the Germans were building up their forces and that Horrocks was sent into battle with insufficient gun power.

A lot of research has been done on the belated attack on the Scheldt estuary (oct-nov '44) where the neglected ULTRA messages played a decisive role on the battles that followed.

The question here relates to Operation Market Garden and the role of SIGINT. Since only in the past years the bulk of ULTRA messages have been studied it becomes clear that some histories have to be rewritten.

For some background research have a look here: at ADA406861. The master thesis has good sources.

I perused hundreds of ULTRA messages at Discovery.nationalarchives.gov.uk to corroborate findings of this thesis and of the publication of Bennet mentioned below.

And some interesting books can be read at archive.org:

  • Ultra in the West by Ralph Bennet. This book prints the numbers of the ULTRA messages next to the text and makes it very easy to relate it.

  • The Ultra Secret by F.W. Winterbotham. This book makes clear that only Army level got access to ULTRA messages. Though this book is not without controversy it is a useful source.

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    Aside from the specific event: Not acting on every bit on intel was part of the whole deception. If the Germans would have noticed their every move being countered, they would know their cipher was broken. As callous as it might seem, sending troops into a known trap without telling them was done.
    – DevSolar
    Commented May 24, 2023 at 11:08
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    @Chuck Which is why I posted a comment, not an answer. Your Q includes several assertions, both explicit and implicit. I did shine a bit of light on one of them. Whether that was "well known" to you or not was not clear from the context you've given.
    – DevSolar
    Commented May 24, 2023 at 13:29
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    @Chuck: If your assertions are well researched - then link to that research. The reception here is unlikely to be pleasant while you make assertions that are supported and not generally known to this community. Further: your opinion on sources may differ from that of members here. Commented May 24, 2023 at 14:11
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    Yet still, welcome to history.SE. We don't bite. We're just a bit suspicious on first-timer questions with strong assertions. ;-)
    – DevSolar
    Commented May 24, 2023 at 14:13
  • @DevSolar - One of the fun imaginative bits of the fictional work Cryptonomicon was an elite commando unit in WWII whose job was to perform raids and other small-scale ops to provide plausible alternative explanations for leaks and losses so that the Axis powers wouldn't have strong evidence their codes were broken. They of course didn't necessarily know that's what they were doing. From their perspective they just got sent on a lot of oddball little raid missions with weird objectives.
    – T.E.D.
    Commented May 24, 2023 at 14:39

2 Answers 2


History.net: Ultra Code Breakers -- The Misunderstood Allied Secret Weapon, emphasis mine:

[T]he British and their American allies evolved a carefully segregated intelligence system that limited the flow of Ultra to a select number of senior officers. The Ultra information dissemination process lay outside normal intelligence channels. For example, the intelligence officers of the Eighth Air Force would not be aware of the existence of Ultra and would therefore not know the duties of the Ultra liaison officers. Those officers, in turn, would forward Ultra intelligence only to the commanders of the Eighth and Ninth Air Forces.


Unfortunately, there were drawbacks. Intelligence is used only if it reaches those who understand its significance. [...] On September 5, Bletchley Park made the following decryption available to Allied commanders in Western Europe:

For rest and refit of panzer formations, Heeresgruppe Baker [Army Group B] ordered afternoon fourth [September 4] to remain in operation with battleworthy elements: two panzer, one-six panzer [Second, Sixteenth Panzer Divisions], nine SS and one nought [Ninth, Tenth] SS panzer divisions, elements not operating to be transferred by AOK [controlling army] five for rest and refit in area Venloo-Arnhem-Hertogenbosch.

Putting this message together with intelligence that soon emerged from the Dutch underground in Holland that SS panzer units were refitting in the neighborhood of Arnhem, Allied commanders should have recognized that Operation Market-Garden had little prospect of success. Unfortunately, they did not put these pieces together, and officers at the highest level at Field Marshal Sir Bernard L. Montgomery’s headquarters who had access to Ultra also failed to draw the correct conclusions.

So yes, there definitely was an Ultra intercept addressing the German forces in the area. And not only an Ultra intercept, but corroborating intelligence from the Dutch underground.

Which allows us to side-step the whole issue of who exactly knew about Ultra, or had Ultra information in hand in the sense of "this is an actual intercept by some undisclosed source" instead of a summary by higher-ups (which would be really hard to source). Information on German troop concentrations was available at some level of Top Brass or another. It turns into a question on "did they act on it, and why (not)".

There might even have been changes to the original plans based on this information, for this reason or that -- again, this will be hard to impossible to source. Either way, Operation Market Garden got the "go ahead".

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    the main reason for the failure of Market Garden was Monty dismissing the information about the German strength around Arnhem out of hand because it came from the Dutch resistance, whom he mistrusted and despised as "amateurs" (and for good reason, the Dutch resistance had long been compromised). Not knowing the source of Ultra, they probably didn't realise it was confirmation of the resistance warnings, rather than reiteration.
    – jwenting
    Commented May 25, 2023 at 4:28

I found the answer in the autobiography of General Horrocks where he clearly stated that it was the sixty-four thousand dollar question where the usual information was to inform him about the enemy troops he would be facing. So the answer to the question is: he didn't have access to ULTRA because he didn't know the German 15th Army was there and that halted him for a precious amount of time. As a consequence he missed all the targets to get to the bridges in time.

Source: Brian Horrocks, Corps Commander, London, 1977, Sidwick and Jackson.

In the mean time, I located 200 ULTRA messages about German troop formations which were distributed under a very select group of British officers, not including General Horrocks.

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    Good answer on the question as-is. Two thing to note, though: 1) Failure of Market Garden was not just a failure of XXX Corps (and General Horrocks). There was -- while meaning absolutely no disrespect to them -- also failure of the 82nd and 101st, which did not take the bridges according to schedule. So it would not be fair to point at the information (not) passed to General Horrocks as the sole issue. (History is never just one thing.) 2) It's important to keep in mind that it isn't about "passing ULTRA on or not", but getting the right picture from all the information available.
    – DevSolar
    Commented Jun 7, 2023 at 7:14
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    (ctd.) Intelligence is usually not about "did I get this piece of information", but "was I able to pick out this piece of information from the background noise of incoming intel". -- Both notes being to the casual reader, who might take the wrong impression from this answer ("Incompetence! If X had just told Y about Z, Market Garden would have been a success." -> History is never that simple.)
    – DevSolar
    Commented Jun 7, 2023 at 7:17
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    Too much focus on the western front. The war was not won or lost in the west. The war was lost for Germany somewhere between late 1941 and mid-1942, it was just a question how long it would take.
    – DevSolar
    Commented Jun 8, 2023 at 12:44
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    Oh, my. As a "professional historian" you should know a) how to not take a differing opinion personally, and b) be aware of relative troop strengths, losses etc., geostrategic realities etc. -- I know the Americans specifically have an inflated opinion of their part in the war. But for every fallen German in the west, there were 10 fallen in the east. You may find that insulting for some strange reason, but it's a documented fact.
    – DevSolar
    Commented Jun 9, 2023 at 14:39
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    And as an aside, you mentioning "the thousands of lives given for freedom" is seriously insulting the millions who died defending their freedom where your patriotism doesn't bother to look. It also disqualifies yourself from being taken seriously, let alone believing your "professional historian with scientific training" shtick.
    – DevSolar
    Commented Jun 9, 2023 at 14:51

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