I suppose it depends how one defines nationalism, wikipedia says:
"Nationalism is an ideology and movement that promotes the interests of a particular nation (as in a group of people)[1] especially with the aim of gaining and maintaining the nation's sovereignty (self-governance) over its homeland. Nationalism holds that each nation should govern itself, free from outside interference (self-determination), that a nation is a natural and ideal basis for a polity, and that the nation is the only rightful source of political power (popular sovereignty)."
I totally agree so far.
"It further aims to build and maintain a single national identity—based on shared social characteristics such as culture, language, religion, politics, and belief in a shared singular history and to promote national unity or solidarity. Nationalism, therefore, seeks to preserve and foster a nation's traditional culture, and cultural revivals have been associated with nationalist movements. It also encourages pride in national achievements, and is closely linked to patriotism. Nationalism is often combined with other ideologies, such as conservatism (national conservatism) or socialism (socialist nationalism) for example."
Mostly agree, but perhaps not necessarily with shared religion or "cultural revivals", depending on what that means.
Orwell's opinion (and unless I'm wrong, it's just that, although it seems to be popularly considered fact nowadays):
"By 'patriotism' I mean devotion to a particular place and a particular way of life, which one believes to be the best in the world but has no wish to force on other people. Patriotism is of its nature defensive, both militarily and culturally. Nationalism, on the other hand, is inseparable from the desire for power. The abiding purpose of every nationalist is to secure more power and more prestige, NOT for himself but for the nation or other unit in which he has chosen to sink his own individuality."
I disagree pretty strongly with this. Personally, I'm not terribly patriotic, and I certainly don't consider my country the best in the world, not even close. But But politically, I consider myself a pretty hardcore nationalist. I believe in sovereignty of all nations, I believe a wide variety of approaches to life and organizing society is (or could be...it's a shame more countries don't seem to learn from each other) very beneficial to mankind, and it concerns me that no one seems to worry about whether we're losing some valuable things with the ongoing cultural homogenization of the planet.