Too much violence is what happened in Europe.
The best of two generations were lost back to back during the world wars, leaving only the weak spineless leaders of today. As a result, Europe is in grave danger of being wiped out by the sheer timidity of those too fragile to go fight. The meek did inherit Europe and now they're screwed.
I do agree that combat often tends to make a species stronger, but the gun rendered that notion obsolete as far as Humans are concerned. A 90 lb weakling can now take out a 200 lb picture of perfection with the slight movement of a finger.