Operation OVERLORD, the opening up of an Allied second front by the invasion of the Normandy beaches in June 1944 was the largest military invasion of all time, but it was preceded by years of industrial scale intelligence collection and dangerous clandestine reconnaissance missions off the French coast. VANGUARD is the untold story of this work, the intelligence machine and covert reconnaissance missions that went into the D-Day planning, such as the signals intelligence intercepts, the agent running operations orchestrated by the 15th Flotilla, to the clandestine work of the X-Craft and COPP (Combined Operations Pilotage Parties) diver teams that scoured the Normandy coast months before the June 1944 deadline. The book pulls together previously unpublished but declassified Top Secret documents, diaries, letters and personal accounts from some of the few remaining veterans who were there.