Egbert's later years saw the beginning of Danish Viking raids on Wessex, which occurred frequently from 835 onwards. In 851 a huge Danish army, said to have been carried on 350 ships, arrived in the Thames estuary. Having defeated King Beorhtwulf of Mercia in battle, the Danes moved on to invade Wessex, but were decisively crushed by Egbert's son and successor King Æthelwulf in the exceptionally bloody Battle of Aclea. This victory postponed Danish conquests in England for fifteen years, but raids on Wessex continued.