...In recent years, [Winston] Churchill’s reputation as “a great man” has evolved as scholars re-examine his doings in many directions and find them wanting — or at least not as consistently and admirably right as he had memorialized them in his own multivolume histories. Throughout his career, Churchill made colossal misjudgments and played a central role in a series of disasters, including the Dardanelles fiasco in World War I, the ruinous intervention in the Russian Revolution, the failed Norway campaign at the outset of World War II, the near-­catastrophic “second Dunkirk” in western France in 1940, the Dieppe raid, the rout on Crete, the fall of Singapore and, most notably for current debates and concerns, the strategic bombing campaign against Germany.

The one unassailable aspect of Churchill’s career up to now has been the series of speeches he made in the darkest days of the war — the period from the German invasion of France in May 1940 until the end of that year. Historians and commentators have long declared that these speeches (recognizable in a few key phrases — “blood, toil, tears and sweat,” “their finest hour,” “so much owed by so many to so few”) inspired the British to fight on despite disaster and the threat of invasion, and are therefore of world-­historical significance. The military historian John Keegan asserted that Churchill’s words “swayed the outcome of the invasion summer.” But, as Richard Toye points out in “The Roar of the Lion,” nearly all the evidence supporting the speeches’ decisive impact on British morale is derived from retrospective accounts — and people’s memories could easily have been shaped by the myth of 1940, a myth promulgated during the war by British propaganda, advanced after the war by Churchill and his supporters, then embraced by Britons who were no doubt flattered by the image it projected of them.

Toye, a professor of history at the University of Exeter, demonstrates the unreliability of such retrospective accounts by citing people who remembered the impact of hearing Churchill on the wireless delivering his post-­Dunkirk speech, with its vow that Britons “shall fight on the beaches. . . .” Yet Churchill never did broadcast that speech; it was delivered, unrecorded, only to the House of Commons. (Churchill did make a phonograph recording of the speech — in 1949.) Indeed, the image of Churchill inspiring a nation huddled around creaking radios is fundamentally wrong — he delivered most of his speeches to Parliament....