One of the most important fault lines between Conservatives (in the general, non-Jewish meaning) and Progressives in America or between Left and Right in Europe, is the issue of the permanence of human nature. Conservatives think it doesn't change, or if so, only at glacial rates, and thus schemes for the betterment of the human condition are a waste of time or worse. Progressives think that human nature can be changed, indeed must, and therefore programs for the betterment of the human condition are the only moral option.
My reading of history shows that neither side is always right. On the one hand, human nature can be shown not to have significantly changed in the past few millenia; on the other hand, there are quite a number of cases where radical change happened.
One of them is the disappearance of militarism from Europe. Seen from our present vantage point, it's hard to shrug off the rather glaringly obvious fact that a few hundred million people in Europe regard political violence in a very different way than their predecessors did in the previous 2,500 years, as far back as historical memory goes on that continent.
The New York Times book section has a review on a new book that promises to be a fine read on this topic: James J. Sheehan, Where Have all the Soldiers Gone?
Of course, the significance of the European development, while vast for Europe, is far from clear for the rest of us, since the change in the nature of the European humans seems not to have happened anywhere else. It's nice to know that such a change is humanly possible, but that doesn't mean all of humanity will follow suit, now or ever.