Wessex was one of the seven major Anglo-Saxon kingdoms (the Heptarchy) that preceded the kingdom of England. It was named after the West Saxons and situated in the south and southwest of England. It existed from the 6th century until the emergence of the English state in the 9th century.
Wessex was, according to the Anglo-Saxon Chronicle (ASC), founded by Cerdic and Cynric, although the specifics given by the ASC are considered to be suspect. The first certain event in Wessex is the baptism of Cynegils around the year 640.
Wessex expanded its boundaries and clashed with its neighbours, including Celtic Dumnonia (essentially modern day Devon and Cornwall, which it eventually came to dominate), and with Mercia. After Egbert defeated Mercia in 825 and the Northumbrians accepted his overlordship in 829 Egbert became the first King of England.
The Burghal system of fortified towns (the "burhs") under Alfred the Great, described in both Asser and the ASC and documented in a unique hidage, helped to prevent the conquest of southern England by the Danish invaders in the 870s. The hidage identifies thirty-three forts, which ensured that no one in Wessex was more than a long day's ride from a place of safety.
Southwark is included, but London fell beyond West Saxon territory. Important West Saxon settlements included old Roman settlements such as Dorchester, or Winchester, which Alfred made the capital in 871, and new burhs such as Wallingford.
There is some evidence that kingship in Wessex was not rigidly hereditary. The strongest candidate from the pool of the senior families was elected or forced his control on the lesser kings. The internal feuding produced by this may have delayed the rise of Wessex as a full kingdom, but this is conjecture.
After the Mercian conquest of its original territories in Gloucestershire and Oxfordshire, its northern boundary was probably the River Thames, and its heartland was the present-day counties of Hampshire, Wiltshire, Dorset, Somerset, and Berkshire.