Complex sentence: Eventually these underkings were defeated and the kingdom reunited, probably by Cædwalla but possibly by Centwine, and a decade after Wulfhere's death the West Saxons under Cædwalla began an aggressive expansion to the east, reversing much of the Mercian advance.
Split into simpler sentences: Eventually these underkings were defeated and the kingdom reunited, probably by Cædwalla but possibly by Centwine. A decade after Wulfhere's death, the West Saxons under Cædwalla began an aggressive expansion to the east, reversing much of the Mercian advance.