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“He just mobilized the English language and sent it to war.”

So says a rueful Viscount Halifax moments before the credits roll in Darkest Hour, an account of Winston Churchill’s installation as prime minister and the fraught initial weeks of his administration leading up to the events depicted in Dunkirk.

Halifax meant that, at that moment, Churchill had convinced Parliament to fight Hitler to the end, though Great Britain lacked almost every resource necessary to win except resolve. Halifax...