Wessex (/ˈwɛsᵻks/; Old English: Westseaxna rīce, "kingdom of the West Saxons") was an Anglo-Saxon kingdom in the south of Great Britain, from 519 until England was unified by Æthelstan in the early 10th century.

The Anglo-Saxons believed that Wessex was founded by Cerdic and Cynric, but this may be a legend. The two main sources for the history of Wessex are the Anglo-Saxon Chronicle and the West Saxon Genealogical Regnal List, which sometimes conflict. Wessex became a Christian kingdom after Cenwalh was baptised and was expanded under his rule. Cædwalla later conquered Sussex, Kent and the Isle of Wight. His successor, Ine, issued one of the oldest surviving English law codes and established a second West Saxon bishopric. The throne subsequently passed to a series of kings with unknown genealogies.

During the 8th century, as the hegemony of Mercia grew, Wessex largely retained its independence. It was during this period that the system of shires was established. Under Egbert, Surrey, Sussex, Kent, Essex, and Mercia, along with parts of Dumnonia, were conquered. He also obtained the overlordship of the Northumbrian king. However, Mercian independence was restored in 830. During the reign of his successor, Æthelwulf, a Danish army arrived in the Thames estuary, but was decisively defeated. When Æthelwulf's son, Æthelbald, usurped the throne, the kingdom was divided to avoid war. Æthelwulf was succeeded in turn by his four sons, the youngest being Alfred the Great.

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