There were the Germanic invaders at first, against whom the late-Roman landholders organized; then there were the Vikings, against whom the mixed late-Roman landholders and Germanic invaders organized; then there were the Muslims (who were a threat earlier than the Vikings, and stayed a threat later than them; remember what "reconquista" means, and what happened to Byzantium); then there were the Protestants, and/or the Catholics, and/or the French... and this is leaving out the alphabet soup of East European enemies like the Balts, the Slavs, the Bulgars, and the Magyars (and the Germans themselves were not part of the medieval world until the 8th century or so AD and the conquest of Germany west of the Elbe by Charlemagne).

This is not to imply that the nobility always _did_ oppose the enemies who they should have been opposing; but it's not at all the case that had there been no feudal militaries, medieval Europe would have been a peaceful place.

(I acknowledge that this is a very late post, and I apologize for that; but it's a common mistake to imagine that medieval European history occurred in a vacuum. The primary culprit here is the, ah, _insular_ way that English-speaking schools teach medieval history...)