>Is there any example of a widely armed society that nevertheless succumbed to classical authoritarianism from the inside?

Plenty. The population of the Weimar Republic was pretty militarized (lots of WWI veterans with combat experience and plenty of activity of "Freikorps" militia https://en.wikipedia.org/wiki/Freikorps#Freikorps_involvemen...). These militant and armed forces largely threw in with the Nazi political movement and contributed to the collapse of the first German democracy and the institutionalization of the Nazi Reich. Just to make one very obvious example.