Charlemagne famously involved in a multi generational war against a people to his east who are called the Saxons. The victory conditions that Charlemagne set up in this war were that the Saxons had to give up their traditional religion or else they were going to die. There were 4,500 Saxons beheaded in a single afternoon at the edge of a river in a town called Verdann because they were allegedly the leaders of one of the many Saxon rebellions against him. And it's hard to know where one ended and the other began.