To my knowledge this question was, and remains, central to most interpretations of Fascism. It is also one which can't really be answered conclusively. As causation is at least very difficult to substantiate.

Most approaches concur that there was something "decisive" in the process of Italian unification - of which Fascism was either a symptom, a manifestation, a degeneration, or a fix. That, in other words, the reasons of the crisis of the Italian liberal system resided either in its "incomplete" or "unfinished" liberalism (which is to say, in comparison to the British and French liberal system), in the inadequacy of the French and British model (in comparison to the German one), or in the fundamental crisis of liberalism to face the necessities of the XX Century.

All these interpretations are actually impossible to disentangle from one another, because they constantly influence each other and exist in a sort of fluid continuum. The usual general reviews - Casucci's Interpretazioni del Fascismo, De Felice's Le interpretazioni del Fascismo, Gentile's Fascismo, storia e interpretazione - provide a summary which may help straighten up the matter somewhat.

What's interesting perhaps is the fact that every interpretation - except perhaps the orthodox Stalinist one - seems to assume that a crisis, a fracture of the Italian national experience occurred at some point. Which is inevitably tied to the Risorgimento.

The War has been for Italy the solution to a deep spiritual crisis – explained in 1927 Giovanni Gentile [Origini e dottrina del Fascismo ; similar albeit more “historical” arguments are present in the contemporary works of Gioacchino Volpe, for instance Genesi del Fascismo 1935] - […] There were within the Italian soul two entirely diverse currents, almost two irreconcilable distinct souls, fighting for almost two decades and acerbly contesting the ground to achieve that conciliation, which always […] results in the victory of one of the two, then called to retain what parts of the vanquished one are still salvageable. [For those reasons more than for the hope of territorial and economical gains] it was necessary to join the war to consolidate with blood this Nation, that had been formed more due to fortunate circumstances than thanks to the will of its children; more thanks to favorable opportunities, than for a coherent effort of internal will of the Italian people, conscious of its unity, of its investment in unity, of its right to unity. Consolidate the Nation, like war alone can do, creating for all citizens one sole thought, one sole feeling, one identical passion and one common hope, a longing day after day shared by all in the same aspiration for each individual life […] to be tied to something that is common to all but transcends the particular interest of each one. […] To create, at last, for good this Nation; in the only way that makes it possible for a spiritual reality to rise: with struggle, through sacrifice.

The idea that the process of Italian unification had been completed only on material grounds, but not on a moral one (the usual reference here being the citation attributed to D'Azeglio, the "make the Italians" one, but likely popularized by Ferdinando Martini in the late XIX Century), was something that - again - most seemed to agree upon, even if very different could be the character of the proposed completion. Certainly Gentile would have objected to the view that the national unification had occurred "without spilling enough blood" - a common refrain of the nationalists, who insisted on the idea that the nation was something "made once and for all", once it was done, at least.

 

The critic towards a “naturalistic” approach to the issue of national identity was a central point of the Italian philosophical and political elaboration within the liberal field before and after the rise of Fascism – and remains, I believe, a valid one. In this context naturalistic is taken to mean a theory that posits the (pre) existence of an external element, of which the objective traits would be a consequence or derivation; rather than taking those elements as coexisting parts of a unity. Following Benedetto Croce one could therefore apply such criticism of naturalism to both the doctrine of historical materialism and the nationalist doctrine of the state. What is more relevant here though is how such an approach can be found in two almost polar opposite view of the national identity. The view that the “Italian identity” as expressed within a social and political formation could be derived and taken back to an original “natural Italian type” to which the social and political forms would optimally have to conform; and the view that the social and political forms in their organization would constitute the complete and “natural” form of the nation, by which the individual experiences and the collective identity would have to be necessarily shaped.

The last line of criticism especially was pointed at the early forms of the Nationalist ideology. As Giovanni Gentile explained (in Aug. 1917 – and he would later rework his sentiments in a more moderate form for obvious reasons) “the Nationalists had a strictly naturalistic concept of nation, that would turn the man into a bizarre creature bound [to it] with a chain, a sort of canis nationalis - a guard dog of the nation - […] nation as a natural, anthropological and ethnographic fact. [But] a nation defined by certain features of the skull, the language, the religion or the complex of historical tradition […] was an idea worth nothing at all [because] history could not be taken as granted, as a presupposed notion […] inherited from one's forefathers as substance of one's nation. Nation was not there, unless one created it […] never taken as a given but always creating it anew.” A criticism that reminds us of what Croce found so unpleasant in the rise of the new nationalisms of early XX Century; writing in 1919: “un autre traditionalisme, de fabrication recente, qui est de l'ultramodernisme et meme du decadentisme; il se dirige d'un cote et tend a donner forme a l'animal national, voire au fauve national”. This rejection of a “naturalistic” take on the matter of national identity was shared by other exponents of the liberal intellectual world – albeit often in a less subtle manner – such as young Piero Gobetti who, within the context of contemporary political debate, described the Nationalists as a small bourgeois movement led by men who had “taken the Italian Risorgimento as an established fact” and attempted to “compensate for their economical inadequacy with the rhetorics of the Motherland”.

The formative moments of a national identity were therefore no longer seen as an active choice, but elevated to an immortal form, whose ideal values and agency had been sublimated into a permanent monumentalization. This observation though, was destined to trouble Fascism as well – that is, those intellectual elements of Italian fascism that shared an inclination for this kind of problem – since the presence of a “natural character”, of a fixed historicized version of the experience of the nation, clashed with any aspiration to the creation of a new man, to re-shape and reform that national identity. Those external, imperishable national values could only work in so far as Fascism was allowed to determine and define their spiritual, social and material form.