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twentieth century. It was a period punctuated by grave and frustrating blanks, particularly during the First World War, when the intensity of bombardment on the fixed fronts of west and east broke cable communication as soon as mobile operations began, and the signal services had not yet succeeded in acquiring compact radios independent of cumbersome power supplies. Intelligence in real time became an impossibility. Commanders lost voice, indeed any other contact, with their forward troops over the shortest distances, and battles degenerated into directionless turmoil.
    Not so at sea. Because of the ready availability of powerful electric current in turbine-propelled warships, radio communication within fleets and between their component units had become standard by 1914. There were difficulties; lack of directionality in contemporary transmitting sets made for interference, so intense in fleet actions that admirals continued to depend upon flag hoists to control their squadrons. Nevertheless, it had become clear by 1918 that the future of naval communications lay with radio.
    Not, however, with radio telegraphy (R/T), as voice broadcasts were denoted to differentiate them from wireless telegraphy (W/T) in Morse code. R/T is insecure; the enemy overhearing it is as well informed as the intended recipient. Protection is possible, through very high-frequency (VHF) directional transmission, as in the Talk Between Ships (TBS) system used by anti-submarine escorts to great effect during the Battle of the Atlantic; but it is intrinsically short-range, as a secure means of speech. The only safe way of sending messages over long distance by radio wave is through encryption, effectively a return to W/T. Paradoxically, therefore, the flexibility and immediacy allowed by voice radio was denied both strategically and, except in limited circumstances, tactically, by its insecurity. While the control of naval warfare became, as the twentieth century drew on, increasingly electronic, through the rise of such derivatives of radio as radar, sonar and high-frequency direction-finding (HF/DF), high-level, long-range communication remained stuck at the level of wireless telegraphy, because of the need to encrypt or encode, the resulting message being sent by Morse.
    That imposed delay, which was why, though the same imperatives should have applied to army and air force communications, soldiers and airmen, caught up in the dynamics of close-range combat when time was too short to encrypt or encode, broadcast freely by voice radio. Tactical codes were developed—the British army’s Slidex, for example—but even Slidex took time. In the cockpit of a single-seat fighter, any form of encryption was impossible. All armies and air forces therefore set up tactical overhearing services, called “Y” by the British, which listened in to the tactical voice radio transmissions of their opponents. 13 Y frequently supplied battlefield intelligence of high value. During the Battle of Britain, for example, the British intercept stations were able to anticipate the warning of air raids supplied by the Home Chain radar stations by overhearing the chatter of Luftwaffe aircrew forming up before take-off on their French airfields.
    Y was nevertheless of limited and local military value. Important radio communications were, from the First World War onwards, always encrypted or encoded, and only a combatant equipped to render secret writing into plain text could hope to do battle on equal terms with the enemy. The competence of the major powers varied, between themselves and also over time. In the forty-five-year struggle between the Germans and the British during the twentieth century, for example, the Germans unknowingly lost the security of their naval codes early during the First World War and did not regain it. The British, partly by capture and partly by intellectual effort, were able to reconstruct their enemy’s book codes in 1914 and thereafter to read