When did the ancient Danes begin to invade Britain and establish the Danish rule?

Just as England was moving towards unification, starting from the end of the 8th century, the Danes, a branch of the Germanic people, began to invade Britain again. The Danes first harassed the coastal areas, then gradually settled down, and established a Danish rule in northeastern Britain in the second half of the 9th century. During the reign of King Alfred of England (reigned from 871 to 899), he actively prepared to build an army and also established a maritime fleet. Alfred relied on these armed forces to finally contain the Danish attack.