But espionage is ubiquitous, and all governments are aware that their opponents — and even their friends and allies — are spying on them. Only rarely do acts of espionage lead to significant tension between states. Even when a spy scandal leads a government to expel another’s diplomats and embassy staff, the furor usually subsides quickly, and staffing levels are restored. 

Espionage has often helped to prevent or reduce tension. During the Cold War, Soviet and East German intelligence agencies recruited large numbers of spies at NATO headquarters and in the West German government. From them, Soviet leaders learned that U.S. and NATO military forces were not gearing up for an attack. Technical means of intelligence-gathering provided further reassurance. The United States and the Soviet Union deployed elaborate systems — including reconnaissance satellites, electronic interceptors and naval eavesdropping devices — to find out what the other was doing with its military, especially its nuclear arsenal. This gave leaders on both sides confidence that the other was not about to launch a surprise nuclear attack. ...