The Vikings were not a major threat to Wessex during Æthelwulf's reign. In 843, he was defeated in a battle against the Vikings at Carhampton in Somerset, but he achieved a major victory at the Battle of Aclea in 851. In 853 he joined a successful Mercian expedition to Wales to restore the traditional Mercian hegemony, and in the same year his daughter Æthelswith married King Burgred of Mercia. In 855 Æthelwulf went on pilgrimage to Rome. In preparation he gave a "decimation", donating a tenth of his personal property to his subjects; he appointed his eldest surviving son Æthelbald to act as King of Wessex in his absence, and his next son Æthelberht to rule Kent and the south-east. Æthelwulf spent a year in Rome, and on his way back he married Judith, the daughter of the West Frankish King Charles the Bald.
When Æthelwulf returned to England, Æthelbald refused to surrender the West Saxon throne, and Æthelwulf agreed to divide the kingdom, taking the east and leaving the west in Æthelbald's hands. On Æthelwulf's death in 858 he left Wessex to Æthelbald and Kent to Æthelberht, but Æthelbald's death only two years later led to the reunification of the kingdom.
In the 20th century Æthelwulf's reputation among historians was poor: he was seen as excessively pious and impractical, and his pilgrimage was viewed as a desertion of his duties. Historians in the 21st century see him very differently, as a king who consolidated and extended the power of his dynasty, commanded respect on the continent, and dealt more effectively than most of his contemporaries with Viking attacks. He is regarded as one of the most successful West Saxon kings, who laid the foundations for the success of his son, Alfred the Great.
At the beginning of the 9th century, England was almost completely under the control of the Anglo-Saxons, with Mercia and Wessex the most important southern kingdoms. Mercia was dominant until the 820s, and it exercised overlordship over East Anglia and Kent, but Wessex was able to maintain its independence from its more powerful neighbour. Offa, King of Mercia from 757 to 796, was the dominant figure of the second half of the 8th century. King Beorhtric of Wessex (786–802), married Offa's daughter in 789. Beorhtric and Offa drove Æthelwulf's father Egbert into exile, and he spent several years at the court of Charlemagne in Francia. Egbert was the son of Ealhmund, who had briefly been King of Kent in 784. Following Offa's death, King Coenwulf of Mercia (796–821) maintained Mercian dominance, but it is uncertain whether Beorhtric ever accepted political subordination, and when he died in 802 Egbert became king, perhaps with the support of Charlemagne. For two hundred years three kindreds had fought for the West Saxon throne, and no son had followed his father as king. Egbert's best claim was that he was the great-great-grandson of Ingild, brother of King Ine (688–726), and in 802 it would have seemed very unlikely that he would establish a lasting dynasty.
Almost nothing is recorded of the first twenty years of Egbert's reign, apart from campaigns against the Cornish in the 810s. The historian Richard Abels argues that the silence of the Anglo-Saxon Chronicle was probably intentional, concealing Egbert's purge of Beorhtric's magnates and suppression of rival royal lines. Relations between Mercian kings and their Kentish subjects were distant. Kentish ealdormen did not attend the court of King Coenwulf, who quarrelled with Archbishop Wulfred of Canterbury (805–832) over the control of Kentish monasteries; Coenwulf's primary concern seems to have been to gain access to the wealth of Kent. His successors Ceolwulf I (821–23) and Beornwulf (823–26) restored relations with Archbishop Wulfred, and Beornwulf appointed a sub-king of Kent, Baldred.