My answer may seem unusual and even offbeat and unconventional, although I have reasons for having the opinions that I have. History was, and still is, my favorite subject, along with CS and programming. I still consider myself an ardent student of history, especially warfare and strategy: the subject has an unending fascination for me, and I feel that there is so much that one can learn from it.
The deepest realization that I have had after reading history is that wars are won and lost, not by the most powerful nations with the most destructive weapons and with the strongest militaries; Rather, the outcome of any war between nations is decided by which of the two sides possesses the best morality. Mind you, when I speak of morality here, I do so in a non-religious and secular sense, the kind of morality that dictates being reasonable, and by “reasonable” I mean the principle that says live and let live.
I have found this principle to be so pervasive and such a consistently running thread throughout the history of nations and warfare, that I am forced to conclude that this almost seems like a Divine Law. And it works all the time, in all wars, across all continents, regardless of era, regardless of the people and races involved, and regardless of their religious beliefs, or even lack thereof. Apparently, God does not play favorites.
A corollary to this principle is that wars are first lost by nations on the moral front; armed defeat inevitably follows. When a nation becomes unreasonable in its conduct with its citizens, when tyranny becomes the norm, and when criminal perpetrators can get away with impunity, in a way it is the first nail in its coffin, no matter how powerful it may seem today. Its defeat may take years or even decades to come about, but it eventually will, just like night follows day.
This principle just works!
Image courtesy: By Diliff - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=39687711