Human beings, as a species, have a tendency to learn and forget the same lessons over and over again. Perhaps it is a flaw in our brains, which seems ill suited to sorting out problems on a multi-generational timescale.
Another school of thought is that violent upheavals, while morally sickening, are replete with silver linings. Without Nazi aggression and the millions sacrificed to end their reign, would we have devoted substantial resources to cracking the atom or space travel (from the Manhattan project and captured Nazi rocket scientists for the moonshot)? Would we have developed international institutions dedicated to peace and stability or human rights legislation to put effect to the words "never again"?
Without the initial whirlwind expansion of Islam, who would have preserved and furthered ancient Greek knowledge while Europeans were stuck in the dark ages?
Without the subsequent European "Age of Discovery", another violent expansion that dominated most of the planet to the benefit of Europeans and their offspring, would we enjoy the global village we have today, where people, resources and ideas move across borders without much of the intensively detrimental repercussions of past migrations (i.e. the barbarian invasions of Roman territories).
In short, either we're too stupid to avoid our past mistakes, or these sorts of things are an inherent part of human social progress. It sounds counter-intuitive, much like how forest fires can actually help to renew a forest, or how sometimes you just need to drink yourself sober. It doesn't mean you don't fight the evildoers of the world, it just means you accept that they will continue to pop up from time to time.
If there is another take on this, I would be happy to hear it.