Long time since I've posted! Let me see if I can address your question.

You might be thinking of George L. Mosse, a German-American academic of the 60's. While not completely given over to psychoanalytic, trauma-derived explanations, he did espouse something very similar to what you have described. Quoting from his essay "Towards a General Theory of Fascism:"

The war experience aided fascism in another, more indirect manner. The front-line soldiers had become immune to the horrors of war, mass death, wounded and mutilated comrades...War had given meaning to their dull and routine lives. The political liturgy of fascism with its countless festivals catered to the same dream of excitement, of taking part in meaningful action.

He goes on to discuss the glorification of war deaths within fascist ideology. G.L. Waite, in his book Vanguard of Nazism, argues something similar, that soldiers accustomed to life in the trenches, and the sense of having a special mission, could not adapt to civilian life. Instead, they turned to Freikorp and paramilitary groups.

That all makes sense to me on an intuitive level. It is just important not to take it too far into the Nazi years. The "old guard" brown shirt and S.A. rowdies represented but a small clique within the Nazi party by the mid-thirties, when Hitler had fully pivoted towards the economic and cultural center of German society, away from the street violence and radicalism. And Christopher Browning in Ordinary Men has shown beyond a shadow of a doubt that there is no use in explaining the Holocaust or even German aggression in World War II by resort only to "German culture." Rather, even the worst perpetrators of crimes against humanity were essentially plain joes only slowly and incompletely convinced to do their grisly duty.

Dirk Schumann encapsulates the post-modern view in his essay "Political Violence, Contested Public Space, and Reasserted Masculinity in Weimar Germany."

The political violence that ultimately pervaded and helped destroy Weimar Germany was not a direct result of the war but was a product of the political "laboratory" of the early years of the republic. After the brief period of regional civil war came to an end in 1921, militant veterans' associations began to emerge that employed a violence aimed not at killing or maiming but at threatening and intimidating their political opponents.

For Schumann, the violence was a sort of ritual whose aim was to control public space - a potent political tactic employed by right wing groups like the Stahlhelm and imitated by the SPD and the Communists. But more than by any obsession with violence, German men were motivated "to restore a man's bruised self-esteem in the private realm of the family."

If you synthesize all of these views, I think you come to this: the former front line soldiers felt powerless and disoriented by the return to civilian life. Joining fascist paramilitary groups gave them a chance to reassert their identity as adventurers and heroes - as men. "Trauma" is too nebulous and personal a concept for historians to analyze completely. In any case, the evidence points to cultural explanations to explain the continuity between the trench and the swastika. After all, plenty of soldiers joined the communist party, or went back to leading normal lives.

I think it comes to how loosely you want to define trauma.

It is a fact that many of the young joining Fascist organization in Italy were war veterans – if only for age opportunities, given that a man in his early twenties in the years 1919-22 had likely served in the military during the war. So that war had been for them a “formative event”. A fact that was noted already by contemporaries; consider for example Togliatti's recollection of the years of fight leading to the rise of Fascism:

This mass was back then formed of ex combatants, of an entire group of unsettled^1 created by the war. We did not understand that at the root of all this there was a social phenomenon proper of Italy; we did not see the deep causes that created it. We did not understand that these ex-combatants, these unsettled, were not isolated individuals but a mass, creating a phenomenon with some features of a class. We did not understand that it was not enough to just send them to hell. Therefore, for example, these unsettled, used to command during the war, wanted to be in charge back home, they were critical of the established powers and posed a whole set of issues that we had to take seriously into consideration

 

At the same time, it would be wrong to necessarily characterize the members of the new Fascist organizations as a bunch of shell-shocked relics, or so much radicalized by war to be unable to settle for anything but violence and conflict. Among them were many that you would call “young professionals”: lawyers, shop owners, etc. And in later years you will find that some of them were able to establish themselves – whether within the party or outside of it – as successful and influential members of their community.

Also, the idea that commitment to violent action needed to be the result of a psychological trauma is in conflict with the way political violence^2 was seen, described, justified or even advocated, both in the press and the political arena, and from both ends of the political spectrum.

 

There is nonetheless a component of truth to the picture – where the war experience favored radicalization; even if this was a multifaceted process that is difficult to discuss in a unitary approach. Also, if we look at the core of the “squadrismo”, the selected few who where actually doing most of the dirty work, we can perhaps trace back their ideal inspiration – and at times personal direct connections – to the actions of the fascist squads in Trieste, starting around the summer of 1920; that featured a heavy representation of former WW1 arditi^3 and mixed anti-socialist ant anti-slavic violence in connection with the deep resentment over the Government's betrayal over Fiume^4.

Still the effect of the War was – I believe – more indirect. Already during the war, and mirroring a sort of irrational enthusiasm which had swept certain areas of Italian society at the time of intervention, a cultivation of non-traditional, non-rational values had begun.

Expressions such as “reducismo” or “trincerismo” began to be widely used both in a positive and negative connotation to describe a new approach to politics in the broadest sense; Mussolini even wrote about “trincerocrazia” or “power/government of the trenches”.

Later, veterans organizations became widespread and contributed – especially during local elections – with their men and their program. So much that often the first Fascist candidates entered the official political life through Combatant organizations^5 .

They were for the most part Privates, or small Officers no longer on duty. Active commitment to a political organization for a career officer would have been inappropriate and was in most cases actively discouraged, which is at the same time a sign that their participation was less likely and that yet it happened^6.

Among the reasons for this significant presence, the fact that, in requesting the help of the squads for protection or strike repression – a feat often performed especially at the beginning of agrarian fascism - the industrial or land owner was more likely to trust someone who had served in the army, whose presence on the other hand gave more legitimacy to the fascist action. Yet again, despite the fact that the “protection squads” were in the early years of fascist violence 1920-21 often led by a veteran, this does not mean that those veterans could rise in the fascist hierarchy. We know in fact that, when the Militia was legalized as an institution of the State in 1923, the Army had to request first for the Militia officers to be trained by the Army and then for the high ranks to be Army men.

 

Overall though, we are already moving away from a simple psychological radicalization brought on by the war experience. To better understand why this radicalization happened we need to give a look to the practical causes of the phenomenon^7 .

Despite the general hardships of the war economy and the incumbent crisis awaiting Italy with the end of the fixed exchange rates, the war years had seen a decrease in the quota of land owned by traditional passive landowners, mostly in favor of more aggressive “funds” - groups of land owners with significant ties to industrial groups or political forces – but also towards small owners and renters. This had brought forward a class of more profit-oriented owners and – in consequence of the increased war profits and of their imminent end – had put a strain on the re-draft of all short term land contracts.

At the same time the end of the war meant a sudden increase in available work force^8 – a work force which also felt entitled to improved life conditions and more direct control of the land; in absence of a more developed political goal. To achieve this, in spite of the lower demands and the lower margins of profit, the land workers banded together, often under delusions of imminent revolution and distribution of the lands, and attempted to force into a single socialist union as many workers as possible; thus antagonizing those who had during the war struggled themselves to elevate over the condition of day-workers^9 . These “slightly above lower class” resented the socialist imposition and ended up in good numbers being the “practical tool” of agrarian fascism, even more than the veterans themselves.

To add to this picture, consider the description given by A. Tasca of the situation in the agrarian region of Emilia-Romagna:

those who do not go through the “lega” (the socialist land workers union) and, taking a lower pay, works all year, take away from the others' living, who retaliate without mercy. They are ostracized; the baker must refuse them bread; they are shunned, as their wife and children... fines are imposed to the owner who gave them work, breaking the [general] work contract. … At the same time there is suspicion against the small owners; there is an attempt to prevent the creation of small funds

In this context – which saw a contemporary sudden triumphant rise of the socialists in administrative elections, especially in agrarian regions – many of the veterans joining the fascist squads were committed to clean up the institutions of the motherland, threatened and abused by the socialists; from the roots to the top. A risk that may have seemed real enough to them to justify violent action^10 . So that their commitment to fascist ideals did not necessarily cross into a conscious political choice – fascism was an opportunity for them to band together and fight the inside enemy, or just to right some perceived wrong in their neighborhood. It was this more “political” choice which had a more significant impact over the long term establishment of the Regime.

Also, we should not discount the economical factors. Wherever the squads acted on behalf of the land owners, there was some form of retribution – either direct or as support to the local “fascio” - which was doubtless a factor in promoting participation from unemployed or partially employed veterans.

 

Overall, we can say that, while veterans played a significant role in the establishment of the fascist squads in 1920-21, and while the “Arditi cult” remained a major feature in the myth of Fascism (even in post war, to the point where fascist and “Arditi” collectibles and memorabilia are largely grouped together); this was not due exclusively to a psychological effect or a direct influence of the first hand war experience but more to a constellation of motives, many of them can be legitimately traced back to the overall effect of the war on Italian society.