The context is High Medieval, (c. 1001—1300). The concept of feudalism is as understood in Europe. I'm not looking for an answer to cover all European empires or dynasties during this period -- which would be too broad (too much work?).
Answers can focus on a selected empire, such as, but not limited to - Angevin Empire, Holy Roman Empire, or even the Caliphate of Córdoba. This question is not about religion, it is focused on high medieval military command and power structure of European empires.
Is the distribution of strong armies controlled by geography, relative strength of king vs court, or by some effectively random factor such as the personality of the ruler, or accidents of time? Or, an entirely consistent plan of medieval leadership, i.e.Kings, Emperor, Caliph?
Feudalism consisted of a set of relationships among the members of the warrior aristocracy. In a feudal relationship, one individual became the vassal of another, more powerful, member of the warrior aristocracy, called the lord. Both the vassal and the lord had specific obligations to one another, and the lord granted a fief (usually land) to the vassal in return for the vassal’s military service.