Currently reading: Charlemagne by Derek Wilson. It was on ok book, kind of a densely written history of Charlemagne. It has three chapters at the end which go through a history of Europe and the occasional reference to Charlemagne in nationalistic and pan-European myth making. They should have been left out.

The Carolingian Empire was an Augustinian one where the competing regions were bound under the martial prowess of Charlemagne and the absolutism of Christianity. Wilson writes:

Anyone who offended against the King or the common good was sinning against God, reprisals under secular law were expressions of divine retribution.

As Wilson notes there was really no idea of secular law in Carolingian Europe. Frankish tradition included that a king divide up his lands between his sons which is probably why Charlemagne's empire did not last beyond his long reign when he died in his 70s.

I did not realise that Charlemagne's capital, for lack of a better word, was in Aachen. I spent time there many years ago and am kicking myself now for not knowing this then. Aachen lies on the German side of the Belgian-German border and not far from Northern France. Which is interesting as the Frankish ethnicity of the 7thC doesn't fit neatly within the modern French and German ideas of nationalism.