The BBC “History” info on the Cambridge Spies, ends with this jaw dropper.
Perhaps one way would to be to consider whether we need intelligence services in the 21st century. They are only a comparatively recent phenomenon (the SIS dates from 1911, the KGB from 1917, and the CIA from as recently as 1947). It could be that nations have been the victim of a vast confidence trick to deceive us about the necessity and the value of spies.
What the heck is up with that question? George Washington used spies at the founding of America. The French kings had quite a spy service. One of the oldest military classics, “The Art of War” has a section devoted to spies. The Byzantine empire has a legendary spy service that allowed then to have a tiny army. During the wars between the Romans and the Carthaginians it seemed that as many cities fell by treachery as by force. Spies are mentioned several times in the Old Testament. Spies have played a vital part in the drama of nations since the dawn of time, and will continue to do so as long as humans want what other humans have.
I wonder if Phillip Knightley really wrote that paragraph, considering he seems to have made a life out of writing spy books.