Henry Landau was a young South African serving with the British Army when he was recruited into what was now known as MI1c by Cumming who was anxious to find someone who spoke Dutch to set up and run train-watching networks in Belgium. Talent-spotted by one of Cumming’s secretaries, Landau was ordered to report to Cumming’s deputy chief, Colonel Freddie Browning, at the MI1c headquarters in Whitehall Court.

He informed me that I had been transferred to the Intelligence Corps, and that as I had been attached for special duty to the secret service, he would take me up to the chief immediately. Up several flights of stairs I went, until I reached the very top of the building. Here, in a room which resembled the stateroom of a ship, I was confronted with a kindly man who immediately put me at my ease. It was the chief, Captain C. He swung round in a swivel chair to look at me – a grey-haired man of about sixty, in naval uniform, short in stature, with a certain stiffness of movement which I later discovered to be due to an artificial leg. After a few preliminary remarks, he suddenly came to the point: ‘You are just the man we want. Our train-watching service has broken down completely in Belgium and north-eastern France – we are getting absolutely nothing through. It is up to you to reorganise the service. I can’t tell you how it is to be done – that is your job.’

Landau went on to run La Dame Blanche, a group of more than a thousand Belgian and French agents who monitored the movement of German troop trains to and from the Western Front. Named after a mythical White Lady whose appearance was supposed to presage the downfall of the Hohenzollerns, it was arguably the most effective intelligence operation of the First World War and according to Cumming produced seventy per cent of Allied intelligence on the German forces.

After the war, Landau was sent to Berlin but soon became bored. He left the Service and during the ’30s wrote a series of books about his time as a spy, all of which were published in the US to avoid prosecution under the Official Secrets Act. This passage is a deliberately disguised account of the work of the best MI1c agent inside Germany, Karl Krüger, not in fact a Dane, but a German naval engineer. Krüger, known variously as TR16, H16, or as here, R16, was recruited by Landau’s boss, the head of MI1c’s Rotterdam bureau, Richard Tinsley, who was a less than scrupulous ex-merchant seaman. One colleague described him as ‘a very rough looking character, rather like the cartoon pictures of convicts.’ Tinsley ran an immensely successful operation with agents across occupied Belgium and France and a number inside Germany itself, of whom Krüger was only the most important.

I KNEW HIM as the Dane. What his name was, or where he came from, I do not know, although I met him several times. Slight of build, fair, with blue eyes, he looked the reserved, well-bred Scandinavian of cultured and professional interests. He certainly did not look the arch-spy that he was. When I came to know him better, however, I realised why he was so successful. He was a marine engineer of exceptional quality; he was a man without nerves, always cool and collected; nothing escaped his austerely competent eye; and he was possessed of an astounding memory for the minutest detail of marine construction.

I read his reports from time to time and marvelled at them. In my opinion, he was undoubtedly by far the most valuable agent the Allies ever had working in Germany. To the chief in England belonged the credit of finding him; at least, I believe so. He became the solitary agent in Germany that our naval section in Holland possessed, but he was all they needed. He covered every shipbuilding yard and every Zeppelin shed in Germany. I can only give a very general survey of his activities, as his reports dealt chiefly with naval matters which were handled by the naval section. But he rendered such outstanding services to our military section as well as his own, and his reports were so brilliant, that I am sure the reader will be interested in the meagre details I can offer.

The key to his success was that he made the Germans believe that he was working for them against us. As a representative of a Danish shipbuilding yard, which was supplying the Germans from time to time with tug boats and marine equipment, he was allowed to travel freely to Kiel, Wilhelmshaven, Hamburg, Bremen, Emden, Lübeck, Flensburg, and other shipbuilding centres. His capable and affable management of company affairs caused such a sea of orders that they were unable to meet the demands. His popularity with German clients and their trust in his apparently candid nature were unbounded. When in due time he applied for a pass to proceed through Germany to Holland it was readily granted, especially in view of his suggestion to the German authorities that he could buy much needed raw material there, and also tug boats and other small craft, which could be purchased as if by his Danish company, but in reality for supply to Germany. He was so successful with his purchases in Holland that regularly, once every three weeks, he was permitted to make the trip.

Little did the Germans know that it was we who were largely responsible for the Dane’s success. Tinsley, because of his shipping connections, was able to give him valuable information as to where he could purchase material, and secure an odd tug boat now and then; and since in the natural course of affairs the British authorities would have protested or prevented such purchases as he made, our lack of action enabled him to return to Germany and ingratiate himself by boasting how successful he had been in covering up his purchases from the British. In this way, he became persona grata with the German authorities, and by bringing back small presents in the way of clothing, food-stuffs, and luxuries which were then unobtainable in Germany, he was able to ingratiate himself with the heads of the shipbuilding yards, and with other German officials.

With his extraordinary memory, he was able to sit down, when in Holland, and write out page after page of reports, giving an exact description of the ships which were under construction or repair, and supplying us with the invaluable naval information on which the admiralty relied absolutely. Every battleship and cruiser has a distinctive silhouette which is as individual as that of a human being. The silhouettes of all the pre-war German warships were known to us, and in these the Dane was so thoroughly drilled that at a distance of several miles he rarely made a mistake in identifying the larger of them.

From him we got full engineering details of the submarines which the Germans were turning out as fast as they could in order to put over their unrestricted submarine warfare campaign. We learned of the number under construction, the repairs which were being made, and, what was very important, the number which were missing. In the Allied defence against submarines, with the use of depth bombs, mines and gunfire, it was often difficult to tell whether these enemy craft had been sunk or had submerged of their own accord.

Long before the Deutschland, the German merchant submarine, was ready for its trip to America, we had received a full description of it from the Dane. From him we also knew of the commerce raiders, which were then being fitted out. He reported the successful return of the Möwe when we thought it was still at large. Through him the British Admiralty got exact details of the German losses at Jutland, and also a minute account of the damage done to some of the ships which returned. In a battle of this description, fought during periods of fog and darkness, it was impossible to make an accurate estimate of losses from direct observation during the action.

A check was kept on all Zeppelin hangars, and here again an account was given of damage done to the ships during their raids on England.

His most sensational report was a detailed description of the big high angle fire guns, weighing several hundred thousand pounds, which several months later fired their three hundred pound shells at Paris from the forest of St Gobain, a distance of 75 miles. Full particulars of the trials which were carried out with these guns firing out to sea from the coast of Helgoland were given by the Dane. The actual damage done by these guns on Paris was relatively insignificant, considering the expense incurred. I believe the total casualties were only about two hundred. The guns were expected to be chiefly effective as a cause of shock and alarm, the mystery of their position and operation being kept up as long as possible. On the mind of the general public the almost magic quality of the great new guns’ power did produce something like panic, but once again the secret service had destroyed for GHQ the element of surprise planned by the Germans. I have often wondered whether the High Command had placed any faith at first in this particular report of the Dane, the facts seemed at the time so incredible.

In addition to these technical details which he brought us, he was able to give us valuable information about political and economic conditions in Germany. Because he was in contact with high officials and officers in Germany, he brought us back the point of view of the men who really knew what was going on, not the opinion of the man in the street, who was told what the German High Command wanted him to believe.

The greatest danger that the Dane ran was in his contact with us, as he never carried any incriminating materials whatever – notes, lists, letters, even special papers or inks – when he was in Germany. With him we employed the same methods that I used in meeting all our agents working in enemy territory. We kept several houses in Rotterdam and in The Hague, which we were continually changing. To reach these places from the office we employed every trick conceivable, such as never going there on foot from the office, never driving up to the door, doubling back on our tracks, and sliding into a doorway to see if we were being followed. As far as possible, we always met these agents at night, not only to avoid recognition, but to prevent the taking of photographs, in which the Germans were expert. A photograph of a man going into a house owned by us was sufficient evidence in the eyes of the Germans for his immediate execution, if they caught him in Germany or the occupied territory.

As soon as the Dane arrived in Holland, he called us by telephone, announcing his arrival under an assumed name known to us. Then we fixed a time of meeting at one of our houses, A, B, C, D, the addresses of which, corresponding to these alphabetical letters, were known to him. In this way all danger arising from a possible overheard telephone message was removed. One of the girls at the telephone exchange might have been in German pay, or the Germans might have tapped our wire, as we once successfully did with theirs until a Dutch telephone linesman discovered it.

On his arrival in the house, the Dane immediately got down to the writing of his report, which he did in German, and this occupied sometimes three or four hours. One by one the German shipbuilding yards, such as Blohm and Voss at Hamburg, the Vulkan Yard at Bredow near Stettin, the Schichau Works at Elbing and Danzig, the Weser Company at Bremen, the Germania Werft, and the Danziger Werft were gone over, and a description was given of the ships on every single slip in these yards, until every shipbuilding yard in Germany was covered.

When I first saw him write out his reports without any notes, in a calm and matter-of-fact manner, I felt convinced he was faking some of them, but I soon learned to respect his statements, when time after time, later on, we received verification of details which he had reported. As soon as his reports were completed, they were rushed to our office on the Boompje for translating, coding, and cabling to London. The Dane generally remained over in Holland for two or three days, sufficient time for the Admiralty to cable back any questions on his reports which they wished answered, or to acquaint him with details of information which they wanted him to secure on his return to Germany.

The Dane continued his work to the very date of the Armistice. He was paid huge sums, far in excess of any of our other agents; and as he was the father of a family, and apparently of high moral character, I am sure he saved his money, which was amply sufficient for him to retire on. In his villa in Denmark today, none of his neighbours suspect, I am sure, the great role this reserved and observant gentleman played during the war. But it was those very characteristics, with a memory truly phenomenal, that made him undoubtedly the master spy.