The business of collection of signals intelligence by the US evolved during and after the two world wars. The National Security Agency was formally created in 1952 to collect and analyse all signal intelligence globally. The Agency’s prime target was the Soviet Union and it worked closely with the British GCHQ. Having concentrated all its resources in covering the Warsaw Pact countries, the Agency seemed to have lost direction after the collapse of the Soviet Union. The adrenaline did not flow in the 1990s and after the September 11, 2001 terrorist attack, the NSA discovered that it did have information about the activities of many of the terrorists and their phone calls, but did not join the dots and share the information. Stung by this, the NSA began to collect all information, even from domestic American sources, without authorisation.

Intelligence leaks of the largest ever number of diplomatic and defence documents by Bradley Manning to Julian Assange of Wikileaks in 2010 created global furore and embarrassment. Pic/Getty Images