In a wider sense, too, the cryptanalytic work only gained meaning through a coordination of many different kinds of activity, of which puzzle-solving, though critical, was but one.
The audacious U-boat captures, the painstaking labour on dreary dockyard lists, the comparisons with aerial reconnaissance and current incidents,
the filing systems whereby duplication of material could be exploited, the engineering of new machinery—all had to be fitted together,
Again, the reading of German signals was but one of the many factors in the Atlantic game which changed in mid-1941.
The assault on Russia drew off the Luftwaffe, and British aircraft were better able to control the Western Approaches.
The U-boats moved out into a new battlefield in mid-Atlantic.
Both escort ships and aircraft were being fitted with radar for short-range submarine detection.
The Huff Duff system for automated, accurate direction finding was beginning to work.
More significantly, trade links were, as in the First World War, bringing about an undeclared American war.
The US Navy was escorting convoys half-way across the Atlantic, and its official neutrality was of advantage to Britain in that U-boats were instructed not to attack American vessels.