Deheubarth was united around 920 by
Hywel Dda out of the territories of
Dyfed, which had come into his possession. Later on, the
Kingdom of Brycheiniog was also added.
Caerleon was previously the principal court of the area, but Hywel's dynasty fortified and built up a new base at
Llandeilo, giving them their name.
After the high-water mark set by Hywel, Dinefwr was repeatedly overrun. First, by the Welsh of the north and east: by
Llywelyn ap Seisyll of Gwynedd in 1018; by
Rhydderch ab Iestyn of
Morgannwg in 1023; by
Gruffydd ap Llywelyn of Gwynedd in 1041 and 1043. In 1075,
Rhys ab Owain and the noblemen of
Ystrad Tywi succeeded in treacherously killing their English-backed overlord
Bleddyn ap Cynfyn. Although Rhys was quickly overrun by Gwynedd and
Gwent, his cousin
Rhys ap Tewdwr – through his marriage into Bleddyn's family and through battle – reëstablished his dynasty's hegemony over south Wales just in time for the second wave of conquest: a prolonged
Norman invasion under the
Marcher Lords. In 1093, Rhys was killed in unknown circumstances while resisting their expansion into
Brycheiniog and his son
Gruffydd was briefly thrown into exile.
Following the death of
Henry I in 1136, Gruffydd formed an alliance with
Gwynedd for the purpose of a revolt against Norman incursions. He took part in
Owain Gwynedd and
Cadwaladr ap Gruffydd's victory over the English at
Crug Mawr. The newly liberated region of
Ceredigion, though, was not returned to his family but annexed by Owain.
The long and capable rule of Gruffydd's son the
Lord Rhys – and the civil wars that followed Owain's death in Gwynedd – briefly permitted the South to reassert the hegemony Hywel Dda had enjoyed two centuries before. On his death in 1197, though, Rhys redivided his kingdom among his several sons and none of them ever again rivalled his power. By the time
Llywelyn the Great won the wars in Gwynedd, in the late 12th century, lords in Deheubarth merely appear among his clients.
conquest of Wales by Edward I, the South was divided into the
historic counties of
Pembrokeshire by the
Statute of Rhuddlan.