Cnut, or Canute, is one of the great "what ifs" of English history. The Dane who became King of England after a long period of Viking attacks and settlement, his reign could have permanently shifted 11th-century England's rule to Scandinavia. Stretching his authority across the North Sea to become king of Denmark and Norway, and with close links to Ireland and an overlordship of Scotland, this formidable figure created a Viking Empire at least...