What happened in Europe is they didn't want to be warlike anymore.
Nothing to do with genes.
And Hitler was able to take advantage of that.
As Churchill said in 1934:
Many people think that the best way to escape war is to dwell upon its horrors and to imprint them vividly upon the minds of the younger generation. They flaunt the grisly photograph before their eyes. They fill their ears with tales of carnage. They dilate upon the ineptitude of generals and admirals. They denounce the crime as insensate folly of human strife. Now, all this teaching ought to be very useful in preventing us from attacking or invading any other country, if anyone outside a madhouse wished to do so, but how would it help us if we were attacked or invaded ourselves that is the question we have to ask.
Would the invaders consent to hear Lord Beaverbrook's exposition, or listen to the impassioned appeals of Mr. Lloyd George? Would they agree to meet that famous South African, General Smuts, and have their inferiority complex removed in friendly, reasonable debate? I doubt it. I have borne responsibility for the safety of this country in grievous times. I gravely doubt it.
But even if they did, I am not so sure we should convince them, and persuade them to go back quietly home. They might say, it seems to me, "you are rich; we are poor. You seem well fed; we are hungry. You have been victorious; we have been defeated. You have valuable colonies; we have none. You have your navy; where is ours? You have had the past; let us have the future." Above all, I fear they would say, "you are weak and we are strong."
What happened in Europe is they didn't want to be warlike anymore.
I don't think we can appreciate the amount of life lost in Europe during those four years. (The loss of life among the males in France impacted census figures in that country through 1990.)
A "warlike society" may very well recover from "deep losses", but it absolutely needs time.
The “warlike societies” of Europe had not truly contended with the Industrial Revolution.
Half a million dead in six months without victory (France in 1914), or 60,000 casualties in one morning to no benefit (the British at the Somme), would likely have made even Caesar or Napoleon blanch.
“they didn’t want to be warlike anymore”
We make fun of the French but that is exactly why they did not want to fight WWII - my wifes great grandfather was killed at Verdun two weeks before his son was born, the monuments to the men, boys and brothers killed that are in the small villages of France is staggering.