The Differences Between Late Medieval and Modern Kingdoms

Late medieval kingdoms were much less coherent in modern states. Kings only exercised royal power alongside or in competition with a whole bunch of other folks claiming power and rights. In the case of an emergency, say a civil war, great nobles could leverage ties to political actors outside their own kingdom. The upshot was that if a king was too young, too incompetent or just plain didn't get along with the nobles, it was easy for things to fall to pieces.