The Battle of Ellandun, fought near Swindon in September 825, is the battle that ended Mercian overlordship in southern England, and established West Saxon dominance. The exact site is unknown, but the most likely of several contenders appears to be near Windmill Hill in the parish of Lydiard Tregoze.
The battle was fought between King Ecgbehrt of Wessex and Beornwulf of Mercia. According to the Anglo-Saxon Chronicle (ASC), it ended in a clear victory for Ecgbehrt, after which Kent, Surrey, Essex and Sussex submitted to him, whilst East Anglia petitioned him for aid against Mercian aggression. Sir Frank Stenton has called it "one of the most decisive battles of English history".
Beornwulf appears to have invaded Wiltshire to take advantage of Ecgberht being occupied with a campaign in the far West, in which (again according to the ASC), "a battle was fought between the Welsh in Cornwall and the people of Devonshire, at Camelford". The ASC is somewhat ambiguous about whether or not the two events were connected, though,
Ellandun marks the point at which Wessex became Top Nation, as Sellars and Yeatman might say, and saw Mercia collapse to roughly half its former size. One could argue that it led to a dilution of Wessex identity as the kingdom expanded to cover virtually all of Southern England, but that just shows the dangers of tying modern Wessex regionalism too closely to ancient history.