A new biography illuminates Admiral William D. Leahy’s role as FDR’s chief of staff, adviser, and friend during World War II

The Second Most Powerful Man in the World: The Life of Admiral William D. Leahy, Roosevelt’s Chief of Staff 

Phillips Payson O’Brien. 544 pp. 

Dutton, 2019. $30

Just four U.S. Navy men have worn the five-star insignia of a full admiral. Among their ranks, William D. Leahy is the least well-known—and, as Phillips Payson O’Brien argues in a new biography, the most under-appreciated. Leahy did not command the U.S. Navy like Ernest J. King, direct naval operations in the Pacific like Chester Nimitz, or win battles at sea like William F. “Bull” Halsey. His behind-the-scenes contributions were evident mostly in the influence he had on American and Allied strategy as chief of staff to President Franklin D. Roosevelt and in presiding over the Joint Chiefs of Staff. In these roles, O’Brien argues, Leahy had an outsized influence on both American policy and Allied strategy. Indeed, he asserts that by 1944, when Roosevelt’s health declined precipitously, Leahy had assumed many of the burdens of the presidency itself—becoming, as the book’s title asserts, “the second most powerful man in the world.”

O’Brien’s is a full biography that covers Leahy’s youth, his active-duty naval career, and his challenging role as American ambassador to Vichy France in 1940-41. But it is in describing Leahy’s wartime role in the Roosevelt White House that O’Brien makes his strongest statement. His book challenges Henry H. Adams’s 1985 biography of Leahy, Witness to Power, which implies, at least, that Leahy’s significance was mainly that he happened to be present when important events unfolded. By contrast, O’Brien’s Leahy is a backstairs Machiavelli who “used the trust of the president…to guarantee that the United States followed his strategic ideas.”  

O’Brien emphasizes how many days Leahy spent with the president, often alone, especially after 1943. He says that FDR found Leahy “efficient and thorough,” “calming and sensible,” “loyal and direct.” Indeed, Leahy’s loyalty may have been his most salient characteristic, one that allowed him, in O’Brien’s words, to assume “a special role in Roosevelt’s life” as both a trusted adviser and close personal friend. Readers can decide for themselves if O’Brien overreaches when, in consecutive chapter titles, he asserts that Leahy became the “Acting President” and that World War II became “Leahy’s War.” 

In the process of elevating Leahy, O’Brien challenges the dominant view that FDR confidant Harry Hopkins was the preeminent member of the president’s official family. He notes that Hopkins began to fail physically at about the same time Roosevelt did, thus creating a void that Leahy filled. O’Brien also actively denigrates U.S. Army Chief of Staff George C. Marshall, portraying him as a relatively minor figure who was driven by “a sense of his own grandeur,” and full of “insecure puffery.” O’Brien goes so far as to argue that without Leahy’s leadership, all of the Joint Chiefs were pretty much helpless. “Roosevelt, Marshall, King, Arnold, and Hopkins,” he writes, “left to their own devices, were a mess.”  

Leahy’s tendency to remain in the shadows sometimes forces O’Brien to rely on circumstantial evidence. More than once, he notes that FDR announced one or another policy decision after being closeted with Leahy; he concludes that the latter “probably,” “must have,” or “very likely” played a critical role in the president’s decision. He very may well have, but Leahy’s own diffidence and loyalty to his boss prevented him from emerging from the shadows to claim a place in history. With this book, O’Brien does it for him. ✯

—Craig L. Symonds is the author of the 2018 book World War II at Sea: A Global History.

This story was originally published in the October 2019 issue of World War II magazine. Subscribe here.