That was Harold Godwinson, not Ethelred, who went down in
history as 'the unready' because he didn't do so well against the Danes. Harold marched 200 miles from
London to York in four days, collecting an army as he went, and destroyed an invading army under Harold Hardrada of Norway, then
had to turn strainght round, march down to
Kent, and fight again a week later. He lost partly due to ill-luck, and partly because the Normans
had got themselves trapped, with their backs against the sea, and probably fought a lot harder than normal. If he'd won, it would have gone down in
history as one of the greatest
military feats in British
history.
Northern
England, the 'Danelaw', was conquered by the Danes in the 9th Century; the last Viking
king of York was Erik Bloodaxe (947-948 and 952-954).
Peace from Viking raids was eventually brought about by Knut (1016-1035); originally
king of
Denmark, he conquered
England, later became
king of Norway, and ruled, at least as a distant overlord, large areas elsewhere in northern Europe including much of Scotland and
Ireland.
His empire collapsed after
his death, and Saxons ruled again in
England (I'm not sure of the exact relationship to the Danish royal house) until 1066.