by Stephen C. Kepher Annapolis: Naval Institute Press, 2020. Pp. xviii, 302+. Illus., maps, diagr., appends., notes, biblio., index. $40.00. ISBN: 1682475085 Preliminary Planning for the “Longest Day” Following the decision at the Casablanca Conference (January 1943) to undertake an invasion of northwestern Europe, British Lt. Gen. Frederick Morgan was made Chief-Of-Staff (COS) to the not yet appointed Supreme Allied Commander (SAC) to begin planning for the operation. In COSSAC, independent scholar and former U.S. Marine Kepher takes the first serious look at Morgan’s life and career, the organization he created, and its work. In its nine months of existence, Morgan’s organization examined and resolved virtually every problem the invasion posed, while creating a closely integrated Anglo-American staff.