It is not easy to trace a neat boundary between national ideas, as they begun to be formulated organically during XVIII century and to find their diffusion and application after the French Revolution, expressed in certain national movements of a more distinctively political and less literary character, and on the other hand the more proper nationalistic ideas and programs which characterized a certain climate in the build up to the Great War.

In some way, one may look at early XX Century nationalism as a movement which was influenced by the idea of nation as it had been expressed and formulated before - but that either consciously or unconsciously aspired and desired to adapt, adjourn update and restore the various national themes to an age where the processes of national unfication, once driven by the principles of national liberalism, appeared - in their failure to achieve that desired "optimum" of social, cultural and political unity - by themselves no longer sufficient to ensure the nation's place among the others.

In its simplest, most basic and simplistic form, this consideration would lead us to share the view that the new nationalism had misunderstood the idea of nation, and transformed it from a positive, liberal value into a convenient excuse to play their war songs - perhaps the nature of national ideas itself is more problematic though...

I am not sure this addresses the central point of your question - and furthermore I am not sure we can define univocally the evolution of European nationalism, since there is one (and at times more than one) nationalism for every nation, so that, for instance and despite the two being related, Italian nationalism and French nationalism had to adapt to markedly diverse national experiences - but I have put together a sort of review of the development of Italian Nationalism during the early years of the XX Century. If your main interest is to previous periods, I can say something about that as well, I suppose.

 

At the turn of the Century, people did realize that Nationalism was a thing, and a thing different from the nation itself, or national sentiment as it had been thought before – it would have been hard to miss. How this realization came to be, and what part of the Nationalist themes inspired or mirrored a general national sentiment is a much more problematic question. We know that the Nationalists were (and always remained) a politically organized minority, rather than a mass movement – we also know that the Nationalist political action exerted an impact on the general population and we can point out a few moments of genuine “nationalistic”, more than “national”, enthusiasm. We can't read minds though, and certain aspects of the “psychology of the masses” are destined to remain obscure to us.

With the XX Century we do begin to witness a “modern” attitude where a political movement is no longer confined to the expression of an intellectual elite and to being the practical outlet of certain interests groups: there is a new need to appeal to the masses, which often becomes a dominating factor, even beyond the actual impact of those appeals, in the definition of the character of the aspiring mass movement. Nor was this idea of appealing to the great forces unleashed by the Century, forces which had been until then confined in a state of political and historical minority, an exclusive of the “subversive” movements – the masses might have become a force of order, if the bourgeoisie proved capable of leading them.

The realization of the new role of the masses in the political, economical, social life of a nation – and of how this role was becoming something different than what the liberal leaders of the XIX Century had hoped, with much more tension, struggles, classes and far less unity and “nation” - didn't really require a large amount of foresight. At the same time, it's certainly true that every nation must have its own nationalism – and perhaps also that every age and time have their own feeling, ideas, sounds and sentimental follies – so that I'll limit myself to the only Nationalism I can conveniently discuss: the Italian Nationalist movement, and specifically within the 1900-1920 time frame.

According to the famed sociologist “of the crowd” and future Nationalist – albeit not for long – Scipio Sighele, who had already offered a contribution on the “collective psychology of parliamentarism” during the last years of the previous Century, there was a sound of the new one (L'intelligenza della folla, 1903):

It is the grandiose collective sound of the crowd that drives our world [...] It has by now become clear that the true protagonist of history [...] has been the people, that is the nameless crowd on which the individualities of the great men worked as if on a brute material to build the architectures of their power [...] and which was, in turn, the dark immense cave whence those greats drew unconsciously their ideas and the means to put them into action. [One can't ignore] this new mighty and irresistible collective factor of social life which, thanks to political freedom, has awakened from its unconsciousness and finds itself free at last to act in the light of public speeches, press, elections and strikes.

It is this symphony of humankind that one has to examine to find the Nationalist themes, and to follow their evolution towards the Great War and through it. If anything, we may expect telling them apart from the others to be quite easy. Of the many groups, organizations and parties which played their role in the Great War, the Italian Nationalists were among those who had been expecting it, and looking forward to it.

The Italian poet and essayist, Giovanni Boine illustrated the nature of war in one of his “military speeches” (Discorsi militari May 1914, collection published in Florence 1915), during the great season of the Italian interventionism:

[Italy was] under the threat of a kind of spiritual gangrene, which had taken root inside every soul and was going sooner or later to spread, leading to either civil war or foreign servitude. [...] What usually cured a nation of similar gangrenes [...] what treated such materialistic and individualistic dispersions, was a common enthusiasm which could violently carry each one away from themselves and turn them, from individuals, or gregarians at most of either family or corporation, into a conscious citizen. And it could be a common misery, some larger event, a new faith, any thing that could drive and stir passions [...] And most of the times, as for an universal law, that thing was war. Comes war and any other voice within the nation becomes silent; the nation apart returns one, leaning towards one goal, as if made into one man alone [...] it found itself united under a sole desire, one mighty desire... the nation takes knowledge of itself.

Silence or not, the Nationalists had accustomed themselves to the sounds of war well before they could experience them first hand; so that they might have earned Vilfredo Pareto's sarcastic remark that “they were at risk of resembling the choirs of Opera, singing their marches while standing on place”. Now that the war had come, they had found themselves more or less where they wanted to be. It remained to be seen whether this would result in their transformation into a proper and substantial political force, whether they would remain a marginal group, no matter how loud and noisy, or whether their impact on the Italian society was even necessarily dependent on their evolution into a mass party.