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.
Of course. But the question is whether genetics were the reason for the change or whether it was a natural moral and emotional revulsion to what happened.
Losses were high in WWI. WWII as well, but I don't think Germany changed after 1945 because the warrior gene pool was diminished. In WWII many unwarlike and unfit people died, and the country had high birthrates before the war, so many of those warlike men who died had already reproduced.
I'd say, yes, it takes time for a warlike society to bounce back, but it also would take a long period of heavy losses to really kill off the "alpha male" gene if there is such a thing. So I think the change was cultural, not genetic.