How did invasions contribute to the beginning of the feudal system?

The feudal system was created, or rather evolved, to meet threats of invasion and other chaos that were more or less constant from the middle of the third century in the Roman Empire and the states that occupied the lands of the Roman Empire after it divided and the West Roman Empire collapsed.

There was no real hope of quick reaction to difficulties by a strong central government as there had been during the times Roman Empire. What developed to replace it was a system in which a much weaker central authority passed that authority off to a group of loyal followers, or vassals, who assembled their own groups vassals to whom they passed authority, and so on to the bottom of the society, which was the serfs.

The people at the higher level than serfs could respond quickly, organize things, and keep the system going, in the face of any threat.

There were problems with this system, especially when the vassals became disloyal. But the thing that destroyed it was that the world outgrew the need for it.