[T]he conventional political wisdom since World War II has identified nationalism with fascism. Fascism and Nazism both were nationalist historical moments, but nationalism is not fascism or Nazism. The US at this moment is arguably the most nationalistic country on earth.
Nationalism is an expression of the intense need for affirmation of national or communal identity as the anchor of individual identity. It is one of the fundamental forces at work in political societies, giving them meaning. It is also one of the "strong" forces in the physics of international relations, if not the strongest. It overrides short-term deviation or distraction. Although it may accompany high-minded internationalism, it does not readily yield to it; the repressed returns. For this reason nationalism has to be accommodated, not stubbornly resisted.
This is the force that has upset the European project and that resists further EU expansion as well as further concentration of executive power. The constitution asks a larger sacrifice of national sovereignty than the French, Dutch, and others are willing to accept.