Covert gathering of information on weapons, military strategy and tactics of an enemy or an ally, in peacetime or in war, may be by highly technical methods, including the use of computers, microphotography, wiretapping, bugging and other devices, or it may be personal and psychological in the sense of blackmail or corruption of key officials.
Espionage systems increased with the growth of nation states in Europe following the [Treaty of Westphalia] (1648). Until after the French Revolution they mainly concentrated on military and not political intelligence. With the Napoleonic wars, the whole nation rather than the elite class became involved in war and so the scope of espionage broadened, but the military basis for espionage activities remained the more important until the 20th century, when intelligence bureaux began to have as much power and autonomy in certain cases as the military or the politicians.
It is suggested that of the 2000 Soviet officials stationed in the USA, 30 to 40% are engaged in espionage at least part of the time.