Such espionage involves covert gathering of information about the capabilities and intentions of foreign governments, about foreign areas where they may have a strategic interest, or about their general position on international relations. Methods include wiretapping, bugging, compiling of political dossiers, microphotography, theft, use of computers, blackmail, intimidation, kidnapping, defection and abuse of diplomatic privilege. It may lead to international tension and cold war and also an extensive domestic political espionage network to ensure the survival of the country's international intelligence unit.
Although Sun Tsu in [The Art of War] wrote of the necessity for espionage systems in 530 BC, it began to evolve as a widespread phenomenon with the rise of nation states in Europe after the [Treaty of Westphalia] in 1648. After the French Revolution and the Napoleonic Wars, the scope of espionage systems expanded from their mainly military basis and started to become more politically orientated. It was not until the 20th century, the rise of national dictatorships, and the increase in the power and autonomy of intelligence bureaux in certain countries that espionage in its modern form evolved.
When military strength decreases, the relative threat tends to increase. This requires more intelligence gathering that focuses on intentions of the potential enemies.