Original sentence: The breakdown of the Carolingian Empire in the later 9th century AD, combined with the relative stabilisation of local European borders after the Christianisation of the Vikings, Slavs, and Magyars, had as one side-effect produced a large class of armed warriors, who were busy squabbling amongst each other
Edited sentence: The breakdown of the Carolingian Empire in the later 9th century AD, combined with the relative stabilisation of local European borders after the Christianisation of the Vikings, Slavs, and Magyars, meant that there was an entire class of warriors who then had very little to do but fight amongst themselves and terrorise the peasant population