Nationalism is the ideology or attitude derived from what has been called the right of self-determination. This can be defined as the capacity and right that every nation or people has to constitute itself into a sovereign State. It can be inferred from the literal reading of this definition that it is inalienable; that is, just as someone should not have the freedom to give oneself into slavery, so a people should not be able to renounce this right forever. In practice, nevertheless, clearly it isn't interpreted with the rigidity of this sort of right/duty. It would be more correct, therefore, to define it as the right that a people has to decide if it wants to constitute itself into a sovereign State.
At first glance the historical nature of this concept stands out: its definition is dependent on the terms “people” and “State”, which take on their present meaning with the French Revolution, as we shall soon see. Therefore, if the parts that make up its definition are a legacy of the French Revolution, so must be nationalism itself. Once this is established, it remains to see whether it is an evil or not.
1. State and People
The State to which every nation has a right is, forgive this obvious remark, the State. That is, the modern State (here the adjective “modern” is descriptive, not limiting; that is, it stresses a fundamental quality of its object, but it is not used to differentiate it from others of its kind). Even though it may share the denomination of “State” with previous incarnations of the political community, the modern State constitutes a completely new reality since its birth with the French Revolution. To give a very brief characterization, it can be described as the State that is an artifact, as opposed to a natural political structure; that is neutral, as opposed to orthodox; and that is particularistic, as opposed to universalistic.  With regards to our present objective, it is important to underline this particularism or exclusivism, which as we will see has a significant impact in the wickedness of nationalism.
When one tries to examine the concept of people or nation, an infinity of definitions soon spring up. What constitutes a people? Is it an objective element, like language, culture, race? If so, which one? Or is it rather something subjective, a willingness to belong? It is useless to go deeper into each option, as from this point of view it is an unsolvable problem. No one who has tried to defend a single criterion has managed to convince everyone else. The problem lies in an erroneous approach that takes as its starting point the idea of the exclusivity of peoples, idea upon which the exclusivity of modern nation-States is founded. When one believes to have discovered a characteristic that exactly describes a concrete people, and nobody else outside that people, it does not take long for reality to prove it wrong. And this is so because the idea that peoples are homogeneous within and exclusive to those without does not belong to reality. It is clear that peoples are not unmoving compartments, so that where one ends another must begin. There are different levels and degrees of identity and community, interacting with variable intensity in diverse territorial extensions. The Balkans are paradigmatic: looking at their demographic complexity, it is easy to see the consequences that the principle of national self-determination has wrought on this territory ever since the downfall of the Austro-Hungarian Empire, which to a great degree repudiated this doctrine.
The national element is a dreadfully inadequate criterion on which to base political entities and draw their borders. Only the French Revolution, taking advantage of a favorable historical situation (the strengthened post-Westphalian State), turned this union and confusion between nation and State into a political and sociological dogma. A brief glance at history –for instance, at the Holy Roman Empire and the many kingdoms that made up the Spanish Monarchy– will suffice to prove that in order to achieve harmonious societies it was not necessary to unite political community to nation (meaning place of birth, devoid of political connotations, in its original meaning). These two historical examples are doubly significant because they lead to the suspicion that the doctrine of nationalism was not only false and fictitious, but that it was also construed in order to serve meticulously-calculated interests. Medieval and later Imperial Spain and the Holy Roman Empire stand out for the limitations that the central political power found in the different parts that constituted the whole. In Spain, the Cortes and fueros or local law of the many kingdoms that made up the Crowns of Castille and Aragon. In the Empire, the power of the princes and electors, highly independent but vassals nonetheless to the Emperor. These limitations were justified in the historical, juridical and institutional differences that gave an identity to each constituent part. Each one maintained a certain original autonomy –not delegated by the central power– and for that reason did not feel threatened, but rather ennobled, by the superior power's work of cohesion. In other words, autonomy and pyramidal subsidiarity were the indispensable requisite for the separation of nation and political form. They were the reason why peoples welcomed the opportunity of political union with others, without fear of losing their identities. This went hand-in-hand with the understanding that every power was ordained to the fulfilment of its ends and thus limited to them, as opposed to the absolutism and singleness of the modern doctrine of sovereignty. Lower local powers, just as the higher powers, had their own spheres: so long as the higher ones did not interfere abusively in the lower ones, giving loyalty to them did not present any problems. It was not a surrender, it was giving everything its due according to its particular nature.
From this necessary correlation between subsidiary autonomy and the existence of multi-national (e.g. Spanish Empire) or partially national (e.g. Republic of Florence) political forms, a suggestive thought soon arises: a Revolution that wanted to destroy intermediate societies and create a despotic direct bond between the individual and the State found the perfect means of doing so by advancing the doctrine of the inseparability of nation and State. It proclaimed the exclusivity of peoples underlying in nationalism in order to bring about a particularistic State in opposition to Christendom's universality.