The relationship between the early English kingdoms and the Welsh/British (I'll use these terms interchangeably) kingdoms were frequently marked by hostility, but there were alliances and periods of co-operation as well. Penda, king of Mercia from 626-655 CE, allied with Cadwallon, king of Gwynedd (625-634 CE) and they both fought against the kingdom of Northumbria. Initially Cadwallon was the more powerful of the two, but this relationship between Mercia and Gwynedd seems to have flipped by 642 when both kingdoms again went to war against Oswald of Northumbria. Some have suggested on the basis of Penda's name that he might have had close familial ties with the British, but this is very hard to establish.

During the 'Mercian supremacy', several northern Welsh kingdoms acted as clients of Mercia, though possibly only in a military capacity. Penda summoned English and Welsh rulers alike for his offensive and defensive campaigns. His successors sought hegemony over England and Wales; Aethelbald (716-757) claimed overlordship over all of southern Britain. Wars between independent Welsh kingdoms and Mercia took place under Coenred (704-709), Aethelbald (716-757), and Offa (757-796). Most of this conflict appears to have taken place in the south of Wales, and the campaigns here under Offa may have subjugated the kingdom of Ergyng. Offa's Dyke, often presented as a defensive measure against Welsh raids, is actually fairly difficult to interpret, since it does not seem to have been maintained after its construction. It has been proposed that it was built to act as a total barrier for a brief period against a resurgent Kingdom of Powys, but this is based on very tenuous evidence, and some English settlements existed to the west of it. It may have been built more for ideological and demarcative reasons than military ones.

Coenwulf (796-821) did conduct military campaigns in the north Welsh kingdoms. He fought in Gwynedd in 816, Dyfed in 818, and Powys in 822; we might see these as comparable to 12th century English campaigns in Wales to support one particular ruler over another and reinforce their hegemony. Certainly it seems the Mercians intended to dominate Wales and generally succeeded, though this authority may have been weakening under Coenwulf and prompted his campaigns.

After Wessex defeated Mercia in 825 and took over its status as hegemon, Ecgebehrt of Wessex (802-839) went to Wales in 830 to ensure their submission. However, Wiglaf's rebellion and re-establishment of Mercia prevented Ecgebehrt from holding on to the area. In 853, Wessex granted Burgred of Mercia (852-874) assistance in subduing the Welsh in his orbit; there seems to have been a division of spheres of influence where Wessex had its 'Welsh' in Cornwall and Mercia had its own sphere in the Welsh peninsula.

Alfred the Great (871-899) defeated the Danes at Edington in 878. At the same time, Rhodri Mawr (844-873?), who had been consolidating his control over northern Wales, was defeated in battle, probably by the Mercians. However, Anarawd (d. 916, Rhodri's son) of Gwynedd defeated Aethelred of Mercia (881-911) in 881 at Conwy, and Mercian control over Wales collapsed. To some extent it was transferred to the Vikings and then Wessex. The Life of Alfred claims that Anarawd, who had originally submitted to Jorvik, agreed to become a subject of Alfred on the same terms as Mercia. Concerned about Gwynedd, Dyfed subjected itself to Wessex as well. However, there is also evidence that Gwynedd, Powys, and Dyfed were all subjects of Aethelflaed, Lady of the Mercians (911-918) and they only accepted the overlordship of Edward the Elder (899-924) after he became king of Mercia in 918. Of the other kingdoms, Gwent and Glywysing in the south-east appear to have already been Edward's subjects by this time and Brycheiniog had been in the Mercian sphere.

I hope this has illustrated the relationship between Mercia, Wessex, and the Welsh kingdoms enough to contextualise the Battle of Buttington in 893. Anarawd had fought off the Mercians and submitted to the Vikings, but Viking pressure caused him to turn to a new overlord, which he found in Alfred. The Viking armies and their East Anglian and Northumbrian auxiliaries failed to penetrate Alfred's hegemony and was forced to move all the way northwest to a location where a joint Mercian, Welsh, and West Saxon army was able to surround and destroy it. Anarawd probably provided at least some of the Welsh manpower here; it must have been uncomfortable to have had the Vikings, originally a useful counterweight and means to throw off Mercian domination, right on the border of Powys.

To conclude, the Welsh at Buttington were likely fighting to repulse the Vikings that had, despite their opposition to the Mercians, proven to be undesirable overlords. They also fought in their capacities as subjects of King Alfred the Great, who they saw as a preferable hegemon to Mercia at that time.

Sources

Charles-Edwards, T. M. ‘Wales and Mercia, 613–918’, in Mercia: An Anglo-Saxon Kingdom in Europe, ed. Brown and Farr, pp. 89–105.

Pratt, D. (2007). The political thought of King Alfred the Great. Cambridge University Press.

Tyler, D. J. (2011). Offa’s Dyke: a historiographical appraisal. Journal of Medieval History, 37(2), 145-161.