John Gray at The New Statesman:
Among the many questions that surround the Cambridge spies, one has occupied historians ever since the scale of their treachery became fully known. Why did they choose to betray their country? Several reason are given why Guy Burgess, Kim Philby, Donald Maclean, Anthony Blunt and John Cairncross – commonly known as the Cambridge Five, though there may have been others – decided to serve the Soviet state. In the 1930s they saw the USSR as the chief bulwark against the advance of Nazism and fascism; in the Second World War, they acted in response to Britain and the USSR being allies; during the cold war, they viewed the United States as the chief threat to world peace. Above all, the spies had an overriding ideological commitment to communism. Acting on this was more important for them than clinging to old loyalties of king and country.
No doubt all of these factors played a part, but they are less than thoroughly convincing. The spies were recruited in the 1930s, when the danger of Nazism was becoming clear; but they continued to serve the Soviet Union after it entered into a pact with Nazi Germany, when many other communist sympathisers fell away, and went on serving the Soviet state after it ceased to be Britain’s ally.