What caused feudalism to come about?

Feudalism is a social and economic system in which people owe duties to their social superiors in return for land and protection. In Europe, feudalism began during the ninth century CE and continued until around the fifteenth century.
The initial impetus for feudalism came from the decline of centralized political authority. The Roman Empire had long since collapsed, but even its self-declared successor, the Holy Roman Empire, found itself unable to control its vast territories, and so increasingly came to rely on nobles to maintain order in the far-flung corners of the realm. Feudalism, then, was a practical attempt to fill a large political and administrative vacuum.
This gave local nobles a significant degree of power in their own fiefdoms. At the same time, they too were expected to serve their own lords, (i.e., emperors and kings), usually in the form of military service. Standing armies weren't a feature of European life at the time, so it was essential for nobles not just to fight for their monarchs, but also to supply them with armed men for wars and conquests.
At the bottom end of the social scale, so to speak, vassals owed feudal allegiance to their lords by working a plot of land that the liege lord had granted them. In return, they were notionally afforded protection. Without any institutions of law or justice in the immediate vicinity, this was an important right. In some cases, vassals were also expected to render military service to their liege lords, forming the tribute of men-at-arms that the lord was in turn expected to provide for his king or emperor.
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