The foundations of Cambridge's contribution to Bletchley Park's extraordinary successes in WWII were laid in the First World War when codebreaking helped both to defeat the U-Boats and bring the United States into the War. The successes of WWII in turn made possible the unprecedented codebreaking alliance between Britain and the US, which still continues. One of the alliance's early Cold War successes was to make the first crucial breakthrough in tracking down the 'Cambridge moles', whom the KGB considered its ablest foreign agents. Christopher Andrew is Professor of Modern and Contemporary History at the University of Cambridge, he is also the Official Historian of MI5, Chair of the British Intelligence Study Group and President of Corpus Christi College, Cambridge. His most recent book is 'The Mitrokhin Archive II: the KGB and the World' (Penguin).