Field Marshal Erwin Rommel commander-in-chief of Army Group B had his headquarters in the castle of the village which was the seat of the Duc de La Rochefoucauld.

Researchers spent almost three years locating survivors of D-Day and over 3000 interviews were undertaken in the United States, Canada, Great Britain, France and Germany.

German officers who assisted with the book included Generaloberst Franz Halder, Hauptmann Hellmuth Lang and General der Infanterie Günther Blumentritt.

[3] Editor Peter Schwed gave the book its title[4] from a comment made by the German field marshal Erwin Rommel to his aide Hauptmann Helmuth Lang on April 22, 1944: "...the first 24 hours of the invasion will be decisive...the fate of Germany depends on the outcome...for the Allies, as well as Germany, it will be the longest day."

In May 2014, publisher Andre Deutsch released the 70th Anniversary Collector's Edition, with the addition of 120 photographs, 30 unpublished removable facsimile documents from the Cornelius Ryan Archive and cover by Brazilian photographer Rodrigo Bressane, depicting an interpretation of the iconic Helmet in Omaha Beach, as seen on the Hollywood adaptation.