On a cold day early in 1942, a young woman reported to a large but anonymous building on the banks of the Mersey in Liverpool. The morning began inauspiciously for Janet Okell, aged just 19: on her first day in a new job, she had forgotten her uniform. After her ID cards were examined and her identity confirmed, she passed through gas-blocking mesh curtains into a concrete-encased bunker – and promptly became lost in a warren of sun-forsaken rooms and corridors. With a gathering sense of dismay she wandered, disorientated, till a passing marine took pity on her and asked who she was here to see. By the time her new boss, Gilbert Roberts, arrived downstairs to collect her, Okell was in tears. She took his handkerchief, blew her nose – and then set to work on a project that changed the course of one of the longest and most important battles of the Second World War.


The faceless exterior of Derby House today belies its former importance: within its reinforced core sat the command centre from which Britain’s war against the U-boats in the Atlantic was orchestrated. Okell had arrived to join the newly formed Western Approaches Tactical Unit (WATU), a select group of ‘Wrens’ – members of the Women’s Royal Naval Service – led by a discharged navy captain tasked with figuring out why German U-boats were sinking so many British ships in the Atlantic, and how they might be stopped.

Operations Room in Derby House during Second World War, Wrens would mark positions of Allied and enemy ship
A Wren marks the positions of Allied shipping in the North Atlantic on a wall of the Operations Room in Derby House, headquarters of Western Approaches Command, known to staff as "The Citadel" (IWM/Getty Images)

The stakes were high. Over the previous two years, Hitler’s submarines had sunk hundreds of Allied ships as they carried food and fuel supplies from the United States – Britain’s transatlantic lifeline. In 1940, no less than 95% of Britain’s fuel came into the country from trading partners and colonies, and 70% of its food supply was imported. Some 68 million tonnes of food and fuel was delivered by a 3,000-strong merchant shipping fleet. Both the British and the Germans knew that if those supply lines were broken, defeat for the British would follow. Prime minister Winston Churchill described merchant shipping as “at once the stranglehold and the sole foundation of our war strategy”.

By 1941, Churchill had come to believe that the outcome of the entire war rested on the outcome of the battle for the Atlantic

German U-boats attacked convoys in groups, like packs of wolves circling flocks of sheep. As the losses mounted – 2,603 British merchant ships were sunk over the course of the war in the Atlantic, as well as 175 naval ships – so, too, did Britain’s fear of the wolfpacks. By 1941, Churchill had come to believe that the outcome of the entire war rested on the outcome of the battle for the Atlantic and, night by night, the battle was being lost. Someone had to figure out what was making the U-boats so effective, and what, if anything, might be done to reverse that success.

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Gilbert Roberts had been discharged from the Royal Navy in 1938 while still in his bounding thirties, following a bout of tuberculosis. After recovering from that illness, Roberts found himself adrift, a retired officer with neither ship nor purpose. But in the first week of 1942, he was told to report to the Admiralty offices with an overnight bag. There he met two of the navy’s most senior officers, who detailed Britain’s ongoing losses in the Atlantic – the extent of which was unknown to most people at the time – and the urgent need to find a solution. Roberts was to take the train to Liverpool and report to Derby House, where he would be given a large room on the top floor. There he was to take charge of a team of young staff and, using any and every means necessary, solve the U-boat problem.

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His task was threefold: discover how the U-boats were operating; develop effective countermeasures; and, finally, teach these new tactics to every captain who sailed the Atlantic. Before Roberts left London that day, he was led into an office in which was waiting Winston Churchill, who said simply: “Find out what is happening, and sink the U-boats.”

Game of consequences

In Liverpool, Roberts was assigned ten Wrens who had been chosen for their aptitude in mathematics and statistics. Armed with little more than balls of string, sticks of chalk and a scroll of canvas, the women set to work designing a game that could closely approximate the chaotic, cat-and-mouse battles being fought a few dozen miles out to sea. With the help of eyewitness reports, they began to restage the Atlantic battles in a game played out on the floor – playful to the casual observer, but serious work of serious consequence.

The Game, as it was to become known, took over the top floor of the building, which came to look like a cross between a school gym and a child’s nursery. The floor was covered in linoleum and divided into painted sectors. On this make-believe ocean, the Wrens moved miniature convoys – model merchant ships and their battleship chaperones – according to directions given by the officers taking part in the exercise. But while the Wrens were permitted a bird’s eye view of the play area, the naval captains were allowed only occasional peeks through holes in canvas booths, arranged at the side of the playing field, positioned to recreate the limitations of visibility at sea.

A U-boat crew watches as an Allied vessel sinks after hitting it with a deck gun, having already torpedoed the shi
A U-boat crew watches as an Allied vessel sinks after hitting it with a deck gun, having already torpedoed the ship. In the first two years of the war, U-boat attacks were devastatingly effective (ullstein bild Dtl./Getty Images)

One Wren would move from table to table, passing useful information between the captains to approximate radio chatter. While each officer was listening to this, another Wren would arrive at his side and pass on urgent information relating to the battle – “ship torpedoed here”, “star shell fired there” – compounding the pressure on the increasingly harangued man. Each turn lasted two minutes.

During the post-mortem that followed each game, all of the players would be treated to a bird’s eye view of the battle. The officers could at last see the tracks of the U-boats drawn on the floor in green chalk, set against the movements of their own ships drawn in white, and learn from the umpires whether or not they had managed to sink any submarines. Often, the officers would realise that they had made numerous dreadful mistakes during the Game, which might have resulted in the loss of their ships in earnest combat.

“Make your mistakes here on the oil-cloth,” said Roberts, “and you won’t make them at sea.” Lessons were learned and the officers left emboldened by the experience, more ready to put the tactics discussed into practice back at sea.

U-boats were slipping between battleships at night and creating havoc from within the ‘flock’, like foxes in a henhouse

By repeatedly playing the Game and referencing what they learned against the testimony of returning captains who had survived U-boat attacks, Roberts and his staff began to unpick ways in which the British fleet had misunderstood the U-boats’ behaviour, and to formulate a set of defensive tactics. One night, after a round of corned-beef sandwiches and coffee, the team experienced a eureka moment. The U-boats, they surmised, were not attacking the convoys from a distance, firing their torpedoes between the escort ships toward their targets. Rather, they were quietly slipping beneath and between the battleships at night and creating havoc from within the ‘flock’, like foxes in a henhouse.

Having exposed this cardinal mistake in British antisubmarine tactics, WATU was able to develop a countermeasure that enabled Royal Navy escort ships to hunt the U-boats based on their suspected hiding place beneath the convoy. Jean Laidlaw, the 21-year-old Wren who handled the statistical analysis, dubbed the operation ‘Raspberry’ – a razz of contempt aimed at Hitler and his U-boats. Raspberry was a revolutionary tactic, and its impact on the war at sea was immediate.

Over just one month in summer 1942, escort ships sank four times as many U-boats as they had in the previous month, beginning an upward trend that would continue, broadly, for the rest of the year. In the months that followed the development of Raspberry, Roberts and the Wrens devised numerous other manoeuvres to suit the expanding variety of wolfpack attacks, each given the name of a fruit or vegetable: ‘Pineapple’, ‘Gooseberry’, ‘Strawberry’, ‘Artichoke’.

Officer class

Some months after her arrival, Janet Okell found herself peeping through a hole in the canvas, looking even more schoolgirlish than usual – like a lookout at the classroom window, poised to signal the arrival of a furious teacher to her chalk-hurling classmates. On the other side of the room, studying a fleet of tiny wooden ships on the lino-covered floor, stood a man of imposing distinction: Admiral Sir Max Horton.

As a young submarine commander in the First World War, Horton had earned a reputation as an unparalleled terroriser of German ships; indeed, his was the first naval kill of that conflict. On return to port, his habit was to signal a successful kill to cheering onlookers by flying the Jolly Roger – a British naval tradition that has continued into the 21st century. Horton’s precocious talent as a submariner propelled him up the ranks, and his experience burgeoned correspondingly; he was later described as “the greatest authority on submarine warfare”.

Okell, in contrast, had never been in a submarine – in fact, she had never been to sea at all. Today, however, there was no timidity. Okell fixed the Admiral – who, from November 1942, was commander-inchief of Western Approaches (the area of the Atlantic directly west of Britain and Ireland) – with a determined stare. Her objective was straightforward: sink Horton’s submarines, hidden from her view, as he slid them around the floor, taking potshots at her ships. Unlike the guessing game Battleship, there was no randomness to her approach. And she was playing not for bragging rights but to prove a point of national importance.

The admiral was sceptical of game-playing, which he saw as a waste of time in Britain’s submarine warfare division when real lives were being lost at sea. (“So what is it that you think you do?” he had said, on first meeting Roberts.) Okell and her fellow Wrens believed their Game held the secret to defeating Hitler’s U-boats. If only she, untrained and barely out of her teens, could sink the decorated submarine commander’s vessels, the admiral would see first-hand that their work was anything but a frivolous waste of time.

British Naval officers peer through holes in a canvas screen as they observe a reconstruction of a battle scenario at WATU headquarters
British Naval officers peer through holes in a canvas screen as they observe a reconstruction of a battle scenario at WATU headquarters. The lessons they learned helped them defend against U-boat attacks (Imperial War Museum)

Now Okell, supported by Jean Laid-law, was pitting her wits – and, more importantly, the WATU team’s innovative tactics – against the submarine ace on the game floor. By that time, Okell had proved herself to be a talented commander of the wooden battleships. So when Horton arrived to play the Game, and took the role of a U-boat commander, she was a natural choice for his opponent. Crucially, doing so could make an important point: if someone so young and inexperienced could use Raspberry to defeat one of the First World War’s most storied submarine captains, the efficacy of the new tactic would be proven.

Horton strode onto the linoleum sea, while Okell stood in her hiding place, and the battle began. Horton’s scepticism dissipated as his opponent successfully sank U-boat after U-boat. Five times Horton attempted to escape the escort ships, and five times Okell and Laidlaw defeated him. On the fifth sinking, Horton, who had become increasingly flustered with each loss, erupted.

“You can see, and I can’t,” he roared. “You just rigged it, didn’t you?”

The submarine ace had been beaten by someone barely out of school, who had never seen battle and wasn’t even an officer

Roberts indignantly explained that no, the Game had not been rigged. Then Horton harrumphingly asked to see who was standing behind the canvas, laying waste to his submarines. To his disbelief, a young woman stepped out. The submarine ace had been beaten by someone who was barely out of school, had never been on an escort ship, had never seen battle and, worse still, wasn’t even an officer.

Nemesis of U-boats

Many of those involved in WATU’s work never spoke of their role in the war, and the group’s contribution is barely remembered in Britain (unlike, for example, the involvement of women in cryptanalysis at Bletchley Park). By the fall of Berlin in 1945, though, the U-boat commanders were intimately familiar with Gilbert Roberts, his team and their tactics. Roberts, who spoke fluent German, was one of the first British naval officers to arrive in Germany following its surrender in May 1945, and on 23 May, he eagerly travelled to the U-boat base in Flensburg, far north Germany. In the main operations room, enlarged and tacked to a wall, he saw his photograph, taken from a magazine article. Beneath the image was a handwritten caption: “This is your enemy, Captain Roberts, Director of anti-U-boat Tactics.”

Between the first week of February 1942 and the last of July 1945, when WATU officially closed, a total of 66 Wrens had completed the course in order to become staff at WATU or its sister units, and some 5,000 naval officers played the wargame run by Captain Roberts and his team during more than 130 courses. Many graduates of the Game credited the battles they waged on the linoleum floor as being instrumental in their subsequent victories during encounters with U-boats at sea.

At war’s end, Admiral Sir Max Horton, who had been defeated on that memorable day in 1942 by two young Wrens, sent a personal signal to all who had served in the unit – a powerful tribute to their quiet but momentous achievement:

“On the closing down of WATU I wish to express my gratitude and high appreciation of the magnificent work of Captain Roberts and his staff, which contributed in no small measure to the final defeat of Germany.”

Simon Parkin is a writer and journalist. His latest book is A Game of Birds and Wolves: The Secret Game that Won the War (Sceptre, 2019)


This article was taken from issue 20 of BBC World Histories magazine