What is nationalism? The word is suddenly and surprisingly important when talking about the times we live in. But we seem to be working without a shared definition.

“You know what I am? I’m a nationalist,” Donald Trump said in an October rally in Houston.

French president Emmanuel Macron slapped back at a commemoration ceremony for World War I in France. “Nationalism is a betrayal of patriotism,” he said. “By saying ‘our interests first, who cares about the others,’ we erase what a nation holds dearest, what gives it life, what makes it great and what is essential: its moral values.”

Macron is not the first to try to make a hard, fast, and rhetorically pungent distinction between nationalism and patriotism. Orwell attempted to do the same in a famous essay. He wrote that patriotism is “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.” On the other hand, “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.”

In the end, Orwell gives a rather unsatisfying account in which all the mental and moral vices of self-interest and self-regard are transmuted and supercharged by their absorption into a nationalistic “we.” Nationalists in his account hold their nations supreme, thereby encouraging themselves to traduce any other people or nation. For Orwell, the patriot prefers this to that. The nationalist privileges us over them. For us, everything, to others nothing.

In his recent book, The Virtue of Nationalism, Yoram Hazony makes a different contrast. His work is not primarily concerned with the moral status or self-deception of individuals, but with the organization of geopolitics. For him the contrast is between nationalism and imperialism. For Hazony, it is the nationalist who respects spontaneous order and pluralism. Imperialists run roughshod over these, trampling local life for the benefit of the imperial center. A border will rein in the ambition of the nationalist, whereas the imperial character rebels against limits. A century ago, in what he called the days of “clashing and crashing Empires,” the Irish nationalist Eoin MacNeil felt similarly. For him, the development of a nation — any nation — had in it “the actuality or the potentiality of some great gift to the common good of mankind.”

It’s difficult to find a consistent definition of nationalism from its critics, meanwhile. Sometimes nationalism is dismissed as the love of dirt, or mysticism about language. Other times it’s the love of DNA.

In the critics’ defense, though, the way nationalism has expressed itself in different nations and different times can be maddeningly diverse. Orwell is tempted to believe the nationalist thinks his nation is best in all things, but much of nationalist rhetoric throughout Europe is a rhetoric of envy or arousal. Nationalists sometimes boast about their nations, but in many circumstances they express despair about their countries; they want to excite their people to achieve more, to take themselves as seriously as some rival national actor takes itself.

I’d like to propose a different way of thinking about the question. When we use the vocabulary of political philosophies, we recognize that we are talking about things that differ along more than one axis. Take Communism, liberalism, and conservatism: The first is a theory of history and power. The second is a political framework built upon rights. The final disclaims the word “ideology” and has been traditionally defined as a set of dispositions toward a political and civilizational inheritance.

I would like to sidestep Hazony’s championing of nationalism as a system for organizing political order globally, a theory that my colleague Jonah Goldberg is tempted to call “nationism.”

My proposal is that nationalism as a political phenomenon is not a philosophy or science, though it may take either of those in hand. It isn’t an account of history. Instead, nationalism is an eruptive feature of politics. It grows out of the normal sentiments of national loyalty, like a pustule or a fever. It could even be said that nationalism is patriotism in its irritated state, or that nationalism recruits the patriotic sentiment to accomplish something in a fit of anger.

In normal or propitious circumstances, national loyalty is the peaceful form of life that exists among people who share a defined territory and endeavor to live under the laws of that territory together. National loyalty attaches us to a place, and to the people who share in its life. Destroying national loyalty would almost certainly bring about the return of loyalties based on creed and blood.

One of the outstanding features of nationalist political movements, the thing that almost always strikes observers about them, is their irritated or aroused character. And it is precisely this that strikes non-nationalists as signaling danger. Republican democracies should be characterized by deliberation. Conservatives distrust swells of passion. Liberals want an order of voluntary rights. But nationalist movements are teeming with powerful emotions: betrayal, anger, aggression.

Therefore, I contend, like a fever, nationalism can be curative or fatal. And, like fevers, it can come and go depending on the nation’s internal health or the external circumstances a nation finds itself in. Foreign aggression and the onset of war will reliably generate nationalist moods and responses. But cultural change can do it too. Maybe a national language falls into sharp and sudden decline under pressure from a more powerful lingua franca. Even something as simple or common as rapid urbanization can be felt to agitate upon a people’s loyalties, and may generate a cultural response for preserving certain rural traditions and folkways. And of course, sometimes nationalism is excited by the possibility of some new possession coming into view, the opportunity to recover or acquire territory or humiliate a historic rival. The variety of irritants explains the variety of nationalisms.

You tend to find a lot of nationalism where there are persistent or large irritants to the normally peaceful sense of national loyalty. Think of western Ukraine, where the local language and political prerogatives have endured the powerful irritant of Moscow’s power and influence in its region, and even in its territory. You find a great deal of nationalism in Northern Ireland, where a lineage of religious differences signals dueling loyalties to the United Kingdom and to Ireland.

Until recently you didn’t find a lot of political nationalism in the United States, because it is a prosperous nation with unparalleled independence of action. But we are familiar with bursts of nationalism nonetheless — for example, at times when European powers threatened the U.S. in the early days of the Republic, during the Civil War and its aftermath, and especially during World War I, which coincided with the tail end of a great wave of migration into the country.

If nationalist political movements are national loyalties in this aroused state, then we must judge them on a case-by-case basis. When non-nationalists notice the irritated and irritable character of nationalism, often the very next thing they say is, “Well, they have a point.”

You would judge a nationalist movement the way you would judge any man or group of men in an agitated state. Do you have a right to be angry about this matter? What do you intend to do about it? How do you intend to do it?

We all do this almost instinctively. We understand that there are massive differences among nationalist projects. In order to assert his young nation’s place on the world stage, John Quincy Adams sought to found a national university. We may judge that one way, whereas we judge Andrew Jackson’s Indian-removal policy very differently. In Europe, we might cheer on the ambition of the Irish Parliamentary party to establish a home-rule parliament in Dublin. That was a nationalist project, but so was the German policy of seeking lebensraum through the racial annihilation of the Jews and the enslavement of Poland, which we judge as perhaps the most wicked cause in human history. We might cheer the reestablishment of a Polish nation after World War I, but deplore some of the expansionist wars it immediately embarked upon.

Nationalist politics tends to be opportunist; it takes other political ideas, philosophies, and forms of mobilization in hand and discards them. Nationalists throughout the 20th century adopted Communism or capitalism to acquire the patronage or weapons to throw off imperial rule, or stick it to a neighbor, for example.

The reemergence of nationalist politics in America and abroad requires us to ask those simple questions. What is bothering them? Do they have a point? What do they want to do about it? Would it be just? In broad strokes I intend to take those questions up.