On the other hand, the intercepts and decrypts in the summers of 1941 and 1942 gave the British government, and Churchill in particular, an accurate picture of Erwin Rommel’s tank strength. That information indicated that the British army had considerable superiority in numbers in the North African theater against the Afrika Korps. These quantitative returns could not indicate, however, such factors as the technological superiority of German tanks and particularly the qualitative edge in doctrine and training that the Germans enjoyed. The intercepts, however, explain why Churchill kept consistent pressure on British Eighth Army commanders to attack the Afrika Korps.

In war, so many factors other than good intelligence impinge on operations that it is difficult to single out any one battle or period in which Ultra alone was of decisive import. Yet there was least one instance in which decrypted German codes did play a decisive role in mitigating enemy capabilities.

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By the first half of 1941, as more and more U-boats were coming on line, the German submarine force was beginning to have a shattering impact on the trade routes on which the survival of Britain depended. The number of of British, Allied, and neutral ships sunk climbed ominously upward.

Through spring 1941, the British had had little luck in solving the Kriegsmarine’s ciphers. But in mid-May 1941, they captured not only a German weather trawler with considerable material detailing settings for naval codes but also a U-boat, U-110, with its cipher machine and all accompanying material. With these seizures, British intelligence gained the navy Enigma settings for the next two months. As a result, the British were able to break into U-boat message traffic at the end of May. Because German submarines were closely controlled from shore, and a massive amount of signaling went back and forth to coordinate movement of ‘wolfpacks (groups of U-boats), the British gained invaluable information ranging from the number of U-boats available, to tactical dispositions and patrol lines. Moreover, once they had two months’ experience reading the naval message traffic, British cryptologists continued breaking submarine transmissions for the next five months. The impact of this intelligence on the Battle of the Atlantic was immediate and crucial.

The dramatic decline in sinkings (compared with those that had occurred during the first five months of 1941) cannot be explained other than that Ultra gave the British a crucial edge over their undersea opponents. No new technology, no increase in escorts, and no extension of air coverage can be credited. Ultra alone made the difference.

Unfortunately for the Anglo-American powers, within two months of the United States’ entry into the war the Germans introduced an entirely new Enigma key setting, Triton, that closed off Ultra decryptions for the remainder of 1942. Thus, right when the vulnerable eastern and southern coasts of the United States opened up to U-boat attacks, Ultra intelligence on German intentions and operations ceased. Direction-finding intelligence was available, of course, but it remained of limited assistance. The Battle of the Atlantic in 1942 was a disaster for the Allies.

When the Germans turned their full attention back to the North Atlantic in early 1943, enormous convoy battles occurred with increasing frequency. Grand Admiral Karl Dnitz had nearly one hundred submarines in the North Atlantic. In opposition, the Allies possessed greater numbers of escort vessels, including escort carriers whose aircraft now made the shadowing of convoys by U-boats almost impossible. Moreover, long-range aircraft from Newfoundland, Iceland, and Northern Ireland were reaching farther into the Atlantic.

At the beginning of 1943, the Allied naval commanders enjoyed one further advantage. Bletchley Park had once again broken the German naval ciphers. That intelligence was not quite as useful as the Ultra intelligence of 1941 that had allowed the British to steer convoys around U-boat concentrations. At times, the Allies were able to carry out similar evasive operations, but the number of German submarines at sea at any given point made such maneuvers increasingly difficult and often impossible. From March to May 1943, the U-boat onslaught badly battered Allied convoys. In May, however, the Allies smashed the U-boat threat so decisively that Dnitz ended the battle. Ultra intelligence played a major role in the turnaround. Because of increases in Allied escort strength and long-range aircraft patrols, one must hesitate in identifying Ultra as decisive by itself. Yet the leading German expert on the Battle of the Atlantic, Jrgen Rohwer, does note:

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