After the Romans left Britannia in the early 5th century, Irish Gaelic tribes colonized Celtic northern Wales. The kingdom of Gwynedd was formed after northern Britons (from as far afield as Strathclyde, Scotland) reclaimed the land and drove out the Irish settlers. Several generations later the kingdom of Gwynedd briefly conquered the Anglo-Saxon kingdom of Northumbria (625–34). While the people of Gwynedd aggressively resisted Anglo-Saxon attempts to seize their lands, several other Welsh petty kingdoms were less fortunate. In 616 Powys, supported by Rhos (a minor kingdom that was usually part of Gwynedd) lost Chester to Æthelfrith of Northumbria. Over the next two centuries, the Welsh kingdoms engaged in internecine warfare amongst themselves, as well as fighting the Anglo-Saxons. Rhodri Mawr, king of Gwynedd, annexed Powys and Seisyllwg (Ceredigion, Gower and Builth) in the 850s and defeated the Vikings at Anglesey in 872. In the early 900s the kingdoms of Dyfed and Brycheiniog were absorbed into Seisyllwg by Hywel Dda, Mawr’s descendant.
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