Although the first American troops had arrived in Britain early in 1942, it was the stream of war materials, tanks and aircraft in particular, that alone could make the reconquest of western Europe conceivable.
That stream had to face the Atlantic U-boat fleet, which by October amounted to 196 in number.
Since 1940 the numbers had trebled, and the sinkings had trebled too.
the U-boats were now accounting for over half the merchant fleet required to supply Britain within a year.
The revived American shipyards were turning out new vessels at top speed, only to have each sunk after three or so ocean voyages.
But now the United States had its own pressing demands in the Pacific.
The total Allied stock of shipping was actually declining, while the number of U-boats was increasing: there would be 212 at the end of 1942, with another 181 on trial.
Fast approaching was the crisis of the western war.
Though more diffuse a crisis than that of the air war of September 1940, it likewise stood to see a make-or-break resolution.
Ten years earlier, Alan had conceived a model of action: 'We have a will which is able to determine the actions of the atoms probably in a small portion of the brain... The rest of the body acts so as to amplify this.'
Now he was one of the clustered nerve-cells, and around him a colossal system which had translated his ideas into concrete form:
a British brain, an electric brain of relays clicking through the contradictions, perhaps the most complex logical system ever devised.
Meanwhile the two years of the reprieve had rendered the rest of the body more prepared and coordinated to use its intelligence.
In the Middle East it was amplifying the dim Morse signals into the sinking of Rommel's army.
But the Atlantic was different; here Eisenhower and Marshall might be cut off on a far greater scale than Rommel unless the brain could awaken into life again.