The Nazis took a lot of their information about Germanic tribes from a short book by the Roman political historian Tacitus called Germania. In this book, Tacitus uses the people of Germania as a foil to expose the decadent moral failings of the Roman people. The barbarians live simple lives, he writes, but they're virtuous; they have no cities, but instead they have good family values; every man does his duty and fights in the army; etc. Tacitus is making an argument something like the 'noble savage' interpretation of indigenous peoples that was pushed in the Enlightenment: they're not civilized, but they have preserved the basic natural virtues that the corruption of civilized life has stripped away from us. Nazis read Tacitus as a literal ethnographic account (rather than the polemical, political tract it actually is), and they found in its pages seeming proof that their ancestors had been simple and barbaric, but also as a consequence virtuous, manly, and above the corruption of the Mediterranean peoples. That was a good thing that made their 'race' strong.

By the 1930s, there was more generally a strong trend among many historians of the late Roman empire that emphasized the degeneracy of the Roman people. The empire was corrupt, soft, full of sexual and other moral vices, and had lost the strength and virtue that made it great during the Republic. The Germanic tribes over-ran the empire because they, being simple barbarians, had escaped these degenerating influences. This, again, makes barbarism out to be a virtue rather than a limitation, and ties in well with the Nazi party's emphasis on rooting out 'degenerate' forms for culture (such as music written by Jews or people of color).

Krebs, A Most Dangerous Book is a great starting place if you want an approachable but thoughtful discussion of how descriptions of Germanic barbarians in books like Tacitus' Germania were re-worked into a Nazi foundation myth.

(I should add that few historians today hold with any of these interpretations of the Roman empire or the barbarians. It is now generally agreed that the Romans and barbarians shared many cultural elements, that the late empire was vibrant and innovative rather than degenerate, and the breakup of the western empire into 'barbarian' kingdoms had much more to do with politics than with cultural differences between barbarians and Romans. The old Nazi ideas of pure barbarians overthrowing degenerate Romans simply don't hold up in the face of the evidence that survives.)