Glory of the Great Dead

Why was Cnut the Great so great ? What inspired a Norman Duke to invade England ?
I have spent years to try to find the answer, and I think I got it !

Information about what is known as the Dark Age, because the spoken language, Norse, could not be written, so the very scarce information available is from contemporary monks, who wrote in lation, and sources like Snorre and Adam of Bremen, who both wrote stories based on oral succession from generations to generations.
You need to take your Sherlock Holmes cap on, but read the book, and judge yourself, whether I have solved the mystery.