is this a coincidence, or could it be the case that in the long run or on the average, societies with a more "sound" set of ethics tend to prevail? I know I'm getting into shaky territory there, but I am not convinced for example that military might has always prevailed.

"Prevail" is a treacherous term. On what timescale? Roman society did quite well for centuries, but they eventually fell. Military might served them well for a long time, and it was eventually what destroyed them in their turn.

I don't know how we'd test the idea that the stronger military always wins. We usually define winning as the test for which military is strongest, so again we're running into definition problems.