The idea of someone trying to absolutely obliterate a medieval kingdom seems like it would be near impossible to do unless it was so small that something close to genocide could wipe it out
Not so much genocide but cultural assimilation. Wales is a pretty good example, as are Burgundy and Flanders. But with the exception of Burgundy in the Dark Ages, these were never really big sovereign Kingdoms even though they'd definitely count as BM-style realms. A map of medieval Western Europe transferred to BM would probably have:
- The County of Toulouse, allied to
- The Kingdom of France, at peace with
- The Duchy of Brittany
- The Kingdom of England, federated with
- The Duchy of Normandy, federated with
- The Duchy of the Aquitaine
- The Kingdom of Scotland
- The Kingdom of Wales
- Ireland (rogue or largely unmanageable)
- The Duchy of Burgundy
- The County of Flanders
The Counts of Toulouse or Flanders were likely to be far more wealthy than the Duke of Brittany or the King of Wales, even if they might technically be vassals of someone else. But the whole idea of a 'realm' is tough to impose on some of these entities because a realm was pretty much whomever happened to get along with whomever else and could claim some cultural or spiritual bond.
