What Shakespeare does for a monarch, it is very hard to undo. Richard III, though softened and cleaned up by assiduous researchers, still limps murderously through the public imagination. And Henry V, even soberly revisited, never quite loses that stirring flap of standards, or the thwack of the Dauphin’s tennis balls deep into the hazard.

Ian Mortimer, who has galloped with panache through the English monarchs from Edward II onwards, promises a different Henry: a king set “on the path to his own self-destruction and the negation of his humanity”. He thinks 1415 marks the crucial moment on that path: the year when Henry, pretending to want peace with France, in fact slowly mobilised the whole of England for war. His last letter to the king of France, on July 28th, just before the invasion, threatened “a deluge of human blood”. He delivered.

This was also a year when rebels were stirring in the kingdom and when religious dissidents, from the Lollards at home to Jan Hus abroad, claimed their own way to salvation. Absolutism and divine right were under attack. The king, who exemplified both, leapt to their defence. Mr Mortimer’s Henry—rigid, unsmiling, religious and obsessive as the year begins—ends it as “a militant Catholic fundamentalist”.

The device of putting just one year under the microscope is a bold one in the medieval context. Medieval sources are scanty. Virtually no royal accounts survive for 1415, and almost no private letters. Mr Mortimer nonetheless luxuriates in what he has: grants to chantries and hospitals, rewards for service, reports from ambassadors, requests for provisioning (all those thousands of longbows, arrows, barrels of beer, sides of beef) and the ceaseless pawning of a large part of Henry’s treasure to pay for his whim of a war. Day by day, the reader is in the thick of things.

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