Not all fascist leaders - even in Italy - were veterans; some of them had only made indirect experience of the Great War. Those who had, nonetheless, overwhelmingly belonged to the "interventionist" front; to those, in other words, who had supported the Italian (or German, but I'll limit myself to Italy here) participation to the conflict, celebrated its significance, and its supreme value as a moment of national redemption.

The idea that the Great War (it's great after all - and great remained, even for those who called it a "deluge") must have appeared not only a dreadful massacre of millions, but also a senseless and miserable slog to all those involved, is the somewhat unfortunate result (for Italy at least) of a "post-fascist" revision in the myth of the Great War conducted for the most part by Marxist historians (but also, to a lesser degree, by Catholics), as well as to the incorporation into the new Republican "national" education of a pre-fascist socialist or popular narrative of the conflict. Their works - focusing, as much as possible, on the miserable and "alienating" (a theme common to both socialists and catholics) experience of the masses - challenged directly the official reading of the Regime. That the war had been the true awakening of the Italian Nation - the moment when the Italian new elites (the future "fascists", then already "interventionists") had led the masses into the fray, bringing to complation, on the one hand, the path to national unification which had begun in the Risorgimento, and on the other, opening a new phase in Italy's history, which was to result in the victorious anti-Bolshevik campaign and in the end of the old liberal State.

It goes without saying that the Fascist interpretation of the Great War is extremely suspect and should obviously not be accepted as is; given how its represents, at best, a retelling of the experience of the conflict from the perspective of a (not indifferent, but certainly not large either) minority, and more properly a form of "self-representation" of the Regime. At the same time - true as it was that large portions of the Italian masses took part to the war in a truly mechanical, alienating way (on this point, there is an immense amount of primary and secondary sources - from Gemelli's physio-psychological studies in 1916-17, to memoirs, to the investigations in letter archives, military records, court proceedings, in Monticone, Procacci, Isnenghi, Rochat, etc.) and that the interventionists themselves had to admit that only a minority, an "elite" had truly "felt" and understood the moral and political significance of the conflict; one should not automatically reject the idea that the Great War represented, at the same time, a true "national" experience.

Mosse's work on the "nationalization of the masses" and the direct ties to the "cult of the fallen soldiers" opened a series of investigations on the necessity - even at individual level - of a "mythology" of the war in order to make sense of the scale and "industrialization" of modern conflict. The solemn ceremony for the "unknown soldier" held in Italy during late October - early November 1921 (the celebration of the Dead, but also of Caporetto's fall and Vittorio Veneto's redemption) is usually cited as the first "mass-national" manifestation in the history of unified Italy; the first instance of the people sharing an "Italian" identity.

This experience didn't reproduce exactly the "interventionist" ideals; but this fact didn't really discourage the future fascist leadership, as it played directly into their self-representation as a new elite, the veritable expression of the trincerocrazia, the new "aristocracy of the trenches", destined to inherit the leadership of the Nation.

All the interventionists – wrote Roger Griffin (The nature of Fascism, 1991) – saw themselves as a vanguard political force creating new values which they felt to be necessary precisely because the vast majority of both the ruling classes and the masses lacked a unifying vision, an epic historical perspective with which to rise above inertia and decadence. The dream of fascists is somehow to inspire an irresistible surge of revolutionary “people power” which they nevertheless co-ordinate and control from above. […]

The supreme effort of the war, thus, served both as proof of the moral, ethical and political superiority of the interventionist front against the "social-neutralist" or "neutral-socialists" front - the "anti-national" "defeatists" to use contemporary terminology - and as a model and inspiration to transfer the modes of behavior and mobilization experienced during the war into peace-time politics and internal conflict.

Again, there is an immense amount of primary sources (and extensive examination of the matter in scholarly literature); so I am only going to pick a few representative ones; even if they are somewhat limited in their scope.

I want to take your lead – wrote an unnamed "Genovese veteran" in Mussolini's Popolo d'Italia (August 4^th 1919), addressing the need to fight back against the "social-neutralist" electoral cartel – to confirm that the vast majority of the combatants felt the same way, had the same thought, already at the time when they were face to face with the enemy. Yes, back then already, we could see this coming, we felt that it would have come to this, not disagreeable after all, necessity. […]

It wasn't the humble infantrymen […] no, it was the others, the wolves of the pus [The Official Socialist Party - the disparaging acronym was the one adopted by "national-interventionists] dressed up as officers […] laying the ground for the hatred, the grudges, stirring the basest of passions, [their action] revealed [its effects] during the retreat [of Caporetto], which turned into a tragedy thanks to those lowlife! […]

And now that the war has been won, and exactly because it was victorious, they […] have thrown out their disguises […] and swear and proclaim with all their voice, with all their anger, selling their souls in order to prove that war was a useless massacre and that there is no difference between the victorious and the vanquished!

The same theme – the fundamental value of the divide marked by the intervention and by the experience of the war between neutralists and interventionists – returned in a brief contribution, courtesy of prof. Emanuele Turchi (who would later that year hold a series of lectures on the figure of Beatrice), and published under the title of “Inside the argument” (July 30^th – page three).

I read the contribution of prof. Arturo Finzi – begun Turchi, referencing a previous published letter (27^th of July), discussing Mussolini's recent “Unity and Action” relaunch of a “democratic” front for the November elections – and I disagree.

The author states: “It's a grave mistake to split citizens into two classes: interventionists and neutralists”.

But, who is to blame for that? Aren't the neutralists those who insist in their hostile attitude towards the interventionists? When the latter triumphed and the inevitable, glorious war was declared, it was a universal duty of all Italian citizens, even of those who didn't want it, to forget any previous dissent and, faced with the supreme test of their Motherland, join all forces together […] so that She could be safe and victorious. Contrary to that, neutralists persisted in their disrupting efforts, they did, in one word, sabotage the war, and now that the war is over, they take advantage of the difficulties which the nation is going through, in order to prove that they had been right all along, while doing nothing to mitigate those difficulties, if not, such as the case of the instigators of those insane strikes, trying to worsen them. […] Most definitely an interventionist can't have anything to share with them.

To add to what /u/Klesk_vs_Xaero outlines, it is important to emphasize that there was a generational aspect within fascism's middle ranks that colored its approach to war. Fascism did draw from disaffected WWI veterans such as Italo Balbo or Ernst Röhm, but it also drew on the generational cohort that was old enough to come of age during the war, but was too young to actually fight in it. Such men included Himmler, Heydrich, and Ciano. The experience of growing up during the war plus the convulsions of the immediate postwar period did contribute to a radicalization among some of the youth in this cohort. The war was something of an abstraction for this youth. They became familiar with war as an experience, but not the worst of its hardships. Sebastian Haffner, who was among this generation, observed:

The truly Nazi generation was formed by those born in the decade between 1900 and 1910, who experienced the war as a great game and were untouched by its realities.

This "uncompromising generation," as the historian Michael Wildt has termed them, provided a solid base for fascist and other far-right movements. In Germany, this generation came of adult age during the early upheavals of the Weimar Republic and the Freikorps recruited heavily among German youth. By the same token, this generation entered into the already quite nationalistic German university system in the 1920s and contributed to the greater radicalization of student life.

Not everyone of this wartime youth generation became an illiberal fascist. The prominent peace activist and journalist Carl von Ossietzky counted among this generation. But the wartime milieu and its aftermath did make this generation more conducive to political radicalization. The wartime youth missed out on the actual war experience but were able to participate in an ersatz war against domestic political enemies. One of the great ironies though for such fascists was that by 1939/40, this wartime generation was often too old to participate in the frontlines of the Second World War.