Of special interest for us is the question, Is fascism a particular social system (dictatorship with such-and-so attributes)? Or was it first and foremost a historically distinctive political and social movement with characteristic values and ideology (violence, nationalism, anti-communism)? Is it a historically specific moment, or is it a systemic development stimulated by some structural feature of modern society (deadlocked conflict between workers and the bourgeoisie)? Crudely -- is fascism a social formation, an ideological complex, a social movement, or a type of government apparatus? And our efforts of explanation will depend on what sort of answer we give to these ontological questions.
These alternative definitions of fascism would give rise to very different explanatory challenges. And in fact, there is a wide variety of explanatory and causal questions that can be considered: Why did the fascist movements arise? Why did they gain a mass following? How did the social realities of capitalism affect the emergence and form of fascism? How important were the particular qualities and ideas of Hitler, Mussolini, or Franco in the evolution of fascism as a social system? Why did fascist dictatorships take the form they did? Why did official and affiliate group violence take the virulent forms that it did? How did fascist governments maintain power? Did these governments gain "legitimacy" and support in their populations? Is there a characteristic "pattern of development" for fascist regimes, or are their political histories deeply contingent on events and persons? Are Germany, Italy, and Spain variants of one social form, or are they simply independent social systems possessing some family resemblances in ideology, propaganda systems, and propensities for violence?
We might also consider whether explanation needs to occur at a lower level -- not "why fascism?" but rather, "why the Iron Guard in Romania", "why this or that feature of Italian fascism", "why this particular feature of Spanish state-military relations in Franco's fascism?". Here the point might be that there are no general or comprehensive explanations of the emergence and development of fascism in all the places it occurred; no common causes that were always or usually instrumental; but rather that each national history needs to be treated in its own terms. But, as Passmore demonstrates, this would be somewhat too skeptical; there certainly were some large international and national forces that facilitated fascist mobilization and seizure of power in many different countries.
The historical phenomena of fascism are interesting and important, because they represented powerful social forces (movements and governments that had great influence on events in the twentieth century). We would like for historical social science to have something substantive and illuminating to say about the causes and trajectory of fascism. And, of course, we would be well advised to notice the warning signs if there are any!
(Another excellent very short introduction from Oxford that is relevant to this topic is Helen Graham, The Spanish Civil War: A Very Short Introduction.)