Complex sentence: Penda had evidently won but had probably forged an alliance with local leaders since the former Dobunnic polity did not immediately become part of Mercia but instead became an allied or client kingdom of the Hwicce.
Split into simpler sentences: Penda had evidently won, but he had probably forged an alliance with local leaders, for the former Dobunnic polity did not immediately become part of Mercia. Instead it became an allied or client kingdom of the Hwicce.