Ask the man in the street how many times England has been successfully invaded and he’ll reply, “Twice: the Romans and the Normans.” Ask a historian, particularly one specialising in constitutional history, and he’ll add a third: William and Mary’s invasion in 1688.

They’re all wrong. There have been at least five successful invasions of England. These three, plus the slow-motion carving out of an England separate from Britain by the Anglo-Saxons and then, fifty years before the one date in English history everyone knows, the Vikings finally succeeded in what they’d been trying to do for the previous 150 years: grab the country.

This long-overdue book is about the Viking invasions that first crippled and then ended the reign of England’s worst-ever king, Æthelred, and the man who finally succeeded him, Cnut. In Denmark, his homeland, Cnut’s name is invariably followed by his appellation, ‘the Great’, but in England, where he spent most of his adult life and where he was buried, he is all but forgotten, his fame as a conqueror eclipsed by the man who followed him, fifty years later. Bartlett’s book seeks to redress that balance and it does a good job of demonstrating what a remarkable king Cnut was, holding together a sea-spanning empire encompassing Denmark, England, Norway and much of Sweden.

As a sea pirate with imperial pretensions, Cnut did all that he could to ensure the history makers of his time – the clerks of the Church – were on his side, as well as doing what he could in later life to atone for the judicious murders of his early life that had made his grasp of the crown more secure. The book is thorough in its exploration of the man and his time, although a little on the bloodless side. This is no fault of the author, but rather inherent in the limited contemporary sources – mainly chronicles and charters – which do not lend themselves to rounded character portraits. Later Norse sagas add colour but the careful historian, and Bartlett is careful, has to be cautious about adding these details to what is a sober assessment of England’s forgotten conqueror.

And the tide story? First related by Henry of Huntingdon in the 12th century (a century after Cnut’s death).