In 829 he conquered Mercia, driving its King Wiglaf into exile, and secured acknowledgement of his overlordship from the king of Northumbria. He thereby became the Bretwalda, or high king of Britain. This position of dominance was short-lived, as Wiglaf returned and restored Mercian independence in 830, but the expansion of Wessex across south-eastern England proved permanent. 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. In 855–856 Æthelwulf went on pilgrimage to Rome and his eldest surviving son Æthelbald took advantage of his absence to seize his father's throne. On his return, Æthelwulf agreed to divide the kingdom with his son to avoid bloodshed, ruling the new territories in the east while Æthelbald held the old heartland in the west. Æthelwulf was succeeded by each of his four surviving sons ruling one after another: the rebellious Æthelbald, then Æthelbert, who had previously inherited the eastern territories from his father and who reunited the kingdom on Æthelbald's death, then Æthelred, and finally Alfred the Great. This occurred because the first two brothers died in wars with the Danes without issue, while Æthelred's sons were too young to rule when their father died. Last English kingdom In 865, several of the Danish commanders combined their respective forces into one large army and landed in England. Over the following years, what became known as the Great Heathen Army overwhelmed the kingdoms of Northumbria and East Anglia. Then in 871, the Great Summer Army arrived from Scandinavia, to reinforce the Great Heathen Army. The reinforced army invaded Wessex and, although Æthelred and Alfred won some victories and succeeded in preventing the conquest of their kingdom, a number of defeats and heavy losses of men compelled Alfred to pay the Danes to leave Wessex. The Danes spent the next few years subduing Mercia and some of them settled in Northumbria, but the rest returned to Wessex in 876. Alfred responded effectively and was able with little fighting to