The story may be a bit more nuanced that that:
From a review of "Genghis Khan and the Making of the Modern World"
"Arguably, however, Genghis Khan and the Mongols were the dominant force that shaped Eurasia and consequently the modern world. Not for what they destroyed – though they wrought much destruction all over the continent – but for what they built. They came close to uniting Eurasia into a world empire, and in so doing they spread throughout it technologies like paper, gunpowder, paper money, or the compass – and trousers. They revolutionised warfare. More lastingly, in the word's of the author: ' ...they also created the nucleus of a universal culture and world system. (...) With the emphasis on free commerce, open communication, shared knowledge, secular politics, religious coexistence, international law, and diplomatic immunity.' ".
Again looking at history, I believe without war we wouldn't have technology. Most of the technology that we take as granted in the medieval period came from the Mongols, who because of war managed to gather together inventors from multiple countries and cultures and put them together. They took Chinese fireworks and Bronze Workers from Eastern Europe (I can't remember the country) and put 1 + 1 together, they built a cannon. They also committed many atrocities, however it was always the victims own fault, many times they killed the messenger, which the Mongol's invented their own postal service so they got pissed off at that and just besieged cities. Other times they were given fare warning, nearly always 'surrender and we'll let you live your lives as you always have', yet people refused. IIRC one time the citizens refused and killed the messenger, this one led to the Mongols diverting a river into the city and then slaughtering everyone.
The book I read (http://www.amazon.com/Genghis-Khan-Making-Modern-World/dp/06...) is a real eye opener. The man merely wanted to protect his own family, but betrayal after betrayal he ended up conquering most of the old world. The only reason he didn't take over all of western europe was because 'middle' europe was so poor they thought it was like what we imagine the third world. Aside from Europe they didn't invade most of Africa and India, this was purely because they couldn't ride their horses and didn't feel like invading.
There's been this undercurrent in archaeology and historiography since the 1960s that interprets the past as a successful of peaceful cultural, technological, and linguistic diffusions and that downplays evidence of conquest, force, and violence. For example, an archaeological belonging to this school that chanced across a layer of an ancient fortress festooned with skulls, arrowheads, and soot would present that layer as evidence of some kind of funerary ceremony instead of applying Occam's razor and suspecting a conquest and sack. It's infuriating.
Reading recent papers on ancient Rome (a personal interest of mine), I find myself stopping, sighing, and saying almost out loud, "Those Germans just moved into the empire as good neighbors? The name 'Vandal' got its connotation for no reason at all? The contemporary historians were just lying propagandists and we can dismiss everything they wrote? Really, dude?". Or, "Indo-European languages replaced almost all indigenous European languages because... cultural diffusion? Before writing? Okay dude." I could go on and on on. These "[migration of] pots not people" people apply the same lens to _everything_, and it's super annoying: what they claim runs counter to what everyone knows deep down is true about human nature.
Fortunately, this "[migration of] pots not people" is no longer tenable. Ancient DNA has opened up a world of new evidence --- see  and . As it turns out, As it turns out, intuitions of the early anthropologists like Gordon Childe --- whom these 1960s-era "pots not people" have not rested from attempting it discredit --- were right after all. Conan the Barbarian is a pretty fair depiction. What you see in the genetic record is wave after wave after wave of conquest and slaughter. Some Y chromosome lineages disappear entirely, while the X chromosome ones survive --- indicating that conquerors (as everyone suspected) basically killed off all the men and took all the women. This pattern is attested in historical times in Asian steppe cultures , and there's no reason to suspect that pre-historic cultures operated any differently. Turns out, they didn't.
The reason Europe speaks Indo-European languages isn't that the ancient inhabitants thought that Proto-Indo-European was neat and adopted it --- it's that the PIE-speaking Yamnaya swept in from the caucuses and slaughtered mostly everyone (especially men) except the Basques. It's not pretty, but that's the way it was.
Moreover, it's important to realize that we all have the latent potential for barbarism inside us. We've been at peace for so long --- almost 75 years --- that we've forgotten that man's natural state is "war of all against all" and that peace is incredibly fragile. Downplaying ancient violence encourages present complacency and endangers our present peace.
 https://www.nature.com/articles/d41586-018-03773-6  https://www.bbc.com/news/science-environment-43701630  https://www.amazon.com/Genghis-Khan-Making-Modern-World-eboo...