@Vthif63
Depends on what kind of ruling and power the king has.
If he has high authority he can easily regulate those under him to prevent infighting, though some allowed it to a point since they themselves already had strong enough forces to rebel any outside countries from trying to take their lands.
Yes, this isn't feudal Japan, and nor was the medieval/victorian system consistent between countries or unchanging. The way they dealt with or allowed taking lands of other nobles was as varied as there were countries.