How did Rome's fall contribute to the rise of feudalism in Europe?

Middle AGES: Europe AFTER THE FALL OF ROME

Because kings were often too weak to repel the invaders, many city dwellers moved into the countryside in hopes of greater safety. As a result of the invasions, and a weak central government, a new social and political system known as feudalism developed

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to act quickly in times of danger to the united states

because you have to act quickly if you want to live/survive.