c.790. Earliest Viking raid on England (Portland, Dorset)
The Anglo-Saxon Chronicle notes that some time in the reign of Beorhtric of Wessex (786-802) three ships of Northmen arrived (at Portland in Dorset), and when the local reeve (Beaduheard of Dorchester) came and tried to lead them to the royal estate, thinking that they were traders, they killed him. And "those were the first ships of Danish men which came to the land of the English": they were by no means the last, as the spectacular raid on Lindisfarne in 793 was to demonstrate. Raids may have been sporadic for the first forty years or so, but they intensified in the 830s (as we can see from more frequent references in the Anglo-Saxon Chronicle): great armies landed in the 860s and proceeded to carve up whole Anglo-Saxon kingdoms, until the 870s and 880s when Alfred of Wessex was the last English king and the only one to successfully see off the invaders. Alfred's defensive workings meant the country was better-prepared when the Vikings returned in the 890s, and over the first half of the 10th century Alfred's descendents won back the rest of the country from Viking lords. Viking raiders would return in the 980s, however, to trouble the kingdom of Æthelred, and finally conquer it in the person of the Cnut the Dane, who became king of England in 1016.
S. Keynes, "The Vikings in England", The Oxford Illustrated History of the Vikings (Oxford: 1997), pp.48-82