Aside from Franklin Roosevelt, no American did more to shape World War II than Admiral William D. Leahy, contends Phillips Payson O'Brien. A stern-faced, salty sailor, Leahy's U.S. Navy career had begun as a cadet; four decades later, he was a trusted friend and advisor to the president and his ambassador to Vichy France—until the attack on Pearl Harbor. Needing one person who could help him grapple with the enormous strategic consequences of the war both at home and abroad, Roosevelt made Leahy the first presidential chief of staff—with more power than the position of today—and when FDR's health failed, Leahy became almost a de facto president.