P. R. C. Groves explains why, in his view, Britain in the early 1930s was possessed by a 'national defeatism', namely the idea that war was immoral and should be banned, and the nations disarmed:
The origins of the malady may be summarized as: the Voluntary System, the Somme and Passchendaele. The sacrifice of the flower of an entire generation -- largely owing to the ineptitude of the military mind, though the responsibility is at the moment immaterial -- implied the loss of a leavening virile influence in our national life. And this loss has vastly increased the influence of the feminists, the clericals, the doctrinaires and the dreamers, because it has decreased the normal healthy counterpoise to it. These well-intentioned idealists argue on a plane which has no relation to reality. Consequently their conclusions are false. The path which they advocate leads not to peace but to perdition. There is but one way to peace, and it lies through justice established and maintained by collective responsibility.1
So there are three parts to this. Firstly, the idea of a "lost generation", the premature deaths of Britain's best and brightest in the Great War. Secondly, the evil results of the loss of their manly influence: feminists and pacifists running riot. Thirdly, his rejection of this in favour of the (presumably virile!) solution of collective security (he endorses Lord Davies' New Commonwealth Society and the right wing of the League of Nations Union).
I tend to agree that it was because of the deaths of so many young men that the idea that war was inherently immoral became popular. But it seems to me (and I realise I'm going out on a limb here :) that this was more because of the fact of their deaths, and the perception that they were sacrificed to no useful purpose, rather than the supposed loss of a generation of masculine leaders. The sheer brute facts of the war, and the disillusionment with its results, were bound to influence what people thought about the use of force in international affairs.
P. R. C. Groves, Behind the Smoke Screen (London: Faber and Faber, 1934), 308. ↩