By Mark Grimsley
When it was finally revealed in 1974 that the Allies had
been reading the encrypted German Enigma transmissions throughout much of the
war—intelligence the Allies called Ultra—historians initially expected the news
to shed light on the conflict's numerous turning points, which it did. They
also thought it would likely emerge as a crucial factor in some of them, which
it didn't. Although Ultra was a major asset to Allied intelligence, few
historians now consider it to have been the decisive factor in any major
operation—with one exception.
When it was finally revealed in 1974 that the Allies had
been reading the encrypted German Enigma transmissions throughout much of the
war—intelligence the Allies called Ultra—historians initially expected the news
to shed light on the conflict's numerous turning points, which it did. They
also thought it would likely emerge as a crucial factor in some of them, which
it didn't. Although Ultra was a major asset to Allied intelligence, few
historians now consider it to have been the decisive factor in any major
operation—with one exception.
The exception was the Battle of the Atlantic, the war's
longest campaign. It began on September 3, 1939, the day Great Britain entered
the war. It did not truly cease until May 8, 1945, the day Germany surrendered.
The battle was crucial. If Britain did not receive enough supplies, it might
not be able to stay in the war. In his memoirs, Winston Churchill confessed,
"The only thing that ever really frightened me during the war was the U-boat
peril."
In the early years, the British relied on warships armed
with depth charges and equipped with radar and sonar to escort their convoys.
The Germans countered with a mix of surface raiders and U-boats, but rapidly
shifted toward the latter. The Kriegsmarine had just 57 U-boats when the war
broke out, but steadily increased this number until, by August 1942, it boasted
300.
The U-boats operated in "wolf packs"—clusters of
several vessels arrayed in a loose chain across the major shipping lines but
within easy supporting distance, so that when one U-boat spotted a convoy the
others could quickly move in to join the attack. Convoys caught by wolf packs
could suffer devastating losses in a matter of hours.
On both sides, radio communications—elaborately encoded to
foil eavesdroppers—played an indispensable role. Unbeknownst to the British,
German cryptanalysts had cracked the Royal Navy codes before the war and had
read British naval traffic for more than two years. Germany, for its part,
utilized Enigma—a complex enciphering machine considered impossible to decrypt
if the correct procedures were used to protect it. Nonetheless, Britain and
France were well aware of Enigma. Polish intelligence had studied it intensely
during the interwar period and just prior to the outbreak of hostilities,
handed the western Allies a working model of the Enigma machine as well as
their extensive knowledge of how to decrypt it.
Using their knowledge of how Enigma worked, along with
higher algebra, gifted insight, and numerous clues ("cribs") from
sloppy encryption operators, British cryptanalysts learned to decipher some
Enigma traffic. The German Wehrmacht and Luftwaffe codes proved relatively easy
to crack because of rampant neglect of good communication procedure. The
Kriegsmarine, however, did much better—which was particularly frustrating
because it was the naval code the British most urgently needed to penetrate.
Britain's first major breakthrough came in May 1941, when
its warships seized a German weather
trawler and, by happy accident, captured a German U-boat. In both cases British
cryptanalysts acquired keys that enabled them to decrypt German naval traffic
fast enough for the information to be operationally useful, especially in terms
of re-routing convoys away from wolf packs.
But in February 1942 the Germans shifted to Triton, a more
complex Enig-ma variant the British couldn't read. The result was a major
increase in U-boat interceptions of convoys and merchant ship losses—over 5.6
million tons between February and November 1942.
All that was about to change, however. In October 1942, two
British destroyers found and attacked U-559 in the eastern Mediterranean Sea. A
barrage of more than 200 depth charges forced the U-boat to the surface. When
the German crew abandoned ship, three British seamen—Tommy Brown, Antony
Fasson, and Colin Grazier—climbed into the U-boat's control room. Making their
way to the captain's cabin, they used a machine gun to open its locked
cabinets, then frantically grabbed the documents they found inside. Brown made
it to safety, but Fasson and Grazier drowned when the U-boat abruptly sank,
never knowing that the documents they had died to secure contained the keys
needed to break the Triton code. It took British cryptanalysts until early 1943
to capitalize on this find. When they did, the results were dramatic.
By that juncture, Allies had all the elements they needed to
wage the Battle of the Atlantic: radar, sonar, improved depth charges, and
long-range aircraft. But the decrypts maximized their usefulness and, moreover,
transformed the nature of the battle. The Allies not only diverted convoys from
wolf packs, but zeroed in on both combat U-boats and the oversized supply
U-boats that enabled the combat boats to greatly extend their time at sea. The
hunters became the hunted.
By May 1943, U-boat losses were so heavy that Admiral Karl
Dönitz withdrew them from the North Atlantic. Although the battle continued at
a reduced tempo, the Allies had effectively won. According to military
historians Allan R. Millett and Williamson Murray, "Ultra's contribution
to the antisubmarine battle now became the most significant intelligence
victory of the war, and the only episode in which intelligence alone had a
decisive impact on military operations."
What would have happened if the Allies had never cracked the
Triton code? It must be acknowledged that the Kriegsmarine could never have
achieved its goal of knocking Britain out of the war. British planners
estimated that Britain needed to import between 9.8 and 11.5 million tons of
supplies per year. The U-boats never came close to sinking that amount. But the
effect would nonetheless have been catastrophic. Unable to divert convoys
around known German wolf packs, the Allies would have suffered much heavier
losses. They would have had much greater difficulty in finding and destroying
German U-boats.
Historian David Kahn is probably on target when he concludes
that a failure to crack the code would have delayed the Allied ground
offensives by several months—and in the case of the Normandy invasion, pushed
it back into 1945. Based on shipping figures, Kahn estimates that the
Mediterranean offensives would have been delayed by three months, and that to
get sufficient tonnage it would have been necessary to transfer vessels from
the Pacific, thereby delaying operations in that theater as well. The increased
number of U-boats (because of reduced losses) would also have made Lend-Lease
supply to the Soviet Union far more problematic. Barring the atomic bomb, the
war might have been extended by as much as two years, until 1947.
The gallantry of three British seamen may therefore have
saved hundreds of thousands of lives.
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