Aachen (in french Aix-La-Chapelle) lies in germany
near Belgium Liege and Nederlands Masstricht.
why that little city ?
it was capital of the Frankish Kingdoms
of that the West Frankish Kingdom bekame later France
While I truely do appreciate somebody who doesnt confuse Frankish and French

...
It originally was part of the Middle Frankish Realm, which later completly fell to the East Frankish Realm, that is, Germany. However, if something alters the decade-long Frankish seperation process, it could well end up in the West Frankish Realm, with all the prestige coming with it - small it may be (but really small only in modern times, in the middle age it was average), but even in the Holy Roman Empire the Emperor had to be crowned there (of course in the last couple of centuuries Frankfurt usurped that role).
So if the Middle Frnakish Realm falls to the West Frankish Realm/France, Aachen could become capital.
Sounds anachronictic. I mean, sure, Petain chose it as seat of government, but always seemed more like a random choice to me. Did Vichy have any role in the middle ages?