Harold Macmillan famously said that Western and Soviet spies helped to prevent Armageddon by telling their governments what the other side was truly planning. Calder Walton expresses discomfort with this viewpoint. In Spies, a pioneering study of espionage from 1917 to the present day, he describes several episodes when rulers in Washington or Moscow were seriously underinformed about what the enemy (or the ‘Main Adversary’, as the United States was known in the Kremlin) intended. The danger of a nuclear holocaust was hideously acute during the 1962 Cuban Missile Crisis. Less well known but equally perilous was the panic stirred up in the Politburo in 1983 by NATO’s Able Archer military exercise. Frantic diplomacy on the part of the US State Department was required to dissuade the Soviets from starting a preventive war.

Walton distinguishes himself from many other writers in the field of intelligence studies by scrutinising the real impact that espionage, whether Western or Soviet, had on international relations. He resists the temptation to assume, whenever a network of spies is unearthed, that it has done irreparable damage to those being spied upon. The reality is usually more mundane. Undeniably, though, some spies, by revealing the identities of those working for the enemy, have brought about the grisly deaths of hundreds of other agents and informers.

During the Cold War, spies’ motivations were often ideological. British traitor Kim Philby was a committed communist. Having escaped into exile in Moscow, he never regretted the bloodshed for which he was responsible. In more recent times, the motives for treason have typically been psychological or financial. American