As others have mentioned, feudalism began as a response to the break up of the Roman empire. While the Romans were in charge, the vast expanse of the empire was one of the safest places in the world. Nobody dared challenge the might of the Roman legions, and inside the Empire, people could travel with relative ease on an extensive network of roads. This aided commerce and the spread of ideas: the rapid spread of Christianity, for example, is credited, in part, to the safe and efficient travel the Romans made possible.
As the Roman legions began to withdraw and the Empire crumbled, lawlessness began to reign. This chaotic situation was no good to anyone, except perhaps the raiding Vikings and the criminal classes—but even they could not prosper long without goods being produced, crops grown, and commerce enabled. The territories of the former Empire needed a new system.
Nobody was strong enough to impose the kind of centralized government the Romans had been able to institute....
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