The historian John Lukacs used to say all the old “isms” of politics were defunct. They’ve become “wasms,” except one — nationalism. Lukacs died five years ago, but the relentless anti-Israel protests on America’s campuses today testify to the truth of his insight.

So does the attempt by authorities in the capital of the European Union bureaucracy to quash a “National Conservatism” conference two weeks ago.

The mayor of Brussels was quick to order police to shut down the conference that brought speakers such as Prime Minister Orban of Hungary and Brexit mastermind Nigel Farage to his city.

Naturally, he claimed he was only doing this to protect everyone from the threat of radical protesters wreaking havoc on the conference and city alike — as if preemptively censoring National Conservatives with government power was the only alternative to letting violent leftists silence them through private intimidation.

Yet some might wonder why a “National” Conservative conference was being held at Brussels in the first place, with a distinctly multinational lineup of speakers from Britain, Poland, Hungary, France, America, and elsewhere.

Critics of National Conservatism — both the conference and the coalition associated with it since the first “NatCon” gathering at Washington, D.C., in 2019 — have often claimed there’s a contradiction in nationalists from different nations working together.

Isn’t that really internationalism? The founder of the National Conservatism conference, the American-Israeli intellectual Yoram Hazony, answered that in the closing chapter of his 2018 book “The Virtue of Nationalism.”

There he relates how in the aftermath of World War II — a conflict widely seen as originating in nationalism, though Mr. Hazony finds it a product of imperialism instead — two opposing responses to the problem of aggression arose.

The European intelligentsia, and eventually many educated Americans as well, chose to reject nationalism in principle and place their hopes in new international institutions: the United Nations, the European Union, and the abstract “international community,” as well as what’s now called the “liberal international order.”

The other response was to reaffirm a defensive and lawful nationalism, above all the effort to create a Jewish state — Zionism.

Mr. Hazony came to perceive the continuing growth of anti-nationalist ideology in elite European and American institutions — including our colleges and universities — as a long-term existential threat to Israel.

Zionism is a form of nationalism, and if all nationalism is bad, then Zionism must also be rejected by the international community and the well-credentialed Westerners who think of themselves as its leaders. 

Yet the opposite was really true; if Israel was to survive as a nation-state, defenders of the Jewish state would have to affirm not only Zionism but nationalism in general. 

And Israel’s best allies wouldn’t be liberal internationalists but rather nationalist conservatives in different places.

Even in democratic Western nations that fought the Nazis in World War II, such as Britain and America, liberals demonized nationalist-minded conservatives as bigots of every kind: xenophobes, racists, and, of course, antisemites. 

Mr. Hazony recognized that the greater antisemitic danger now came from the left — the radical activists in the streets and the genteel bureaucrats in control of institutions like the European Union and U.N. agencies.

His vision has been vindicated in the years since he published “The Virtue of Nationalism”: 

Not only has the left shown its antisemitic as well as anti-Zionist inclinations, but the nationalist right in much of Europe and elsewhere has proved to be strongly supportive of Israel in its time of crisis.

Nationalist leaders such as Prime Minister Meloni of Italy and the Netherlands’ Geert Wilders, not to mention President Trump and Mr. Orban, are staunchly pro-Israel. 

Just as important, in Mr. Hazony’s analysis, they are in favor of the nationalist principle that makes Israel possible.

On the other side of the ledger, the same frenzied students and cold-blooded bureaucrats who think Israel is worse than Hamas think Western nations are exceptionally wicked in comparison to the rest of the world. 

Today’s protests against Israel are part of a larger campaign against the nation-state itself: against national borders, sovereignty, the right of self-defense, cultural continuity and assimilation, and well-defined citizenship.

Leftists long for a post-national world of administrative zones — not nations in any meaningful sense — overseen by enlightened experts whose authority doesn’t rest on the consent of any specific people, but who are ritualistically maintained in office by a well-managed fluid voting pool of identity constituencies and broken individuals.

The French political scientist Pierre Manent argues that without nations, without some specific people in a particular place, there can be no democracy.

Nationalism has its defects, and National Conservatism may not always remedy them.

Yet if there’s going to be any democracy in the 21st century — in America, Europe, Israel, or anywhere — there must be nations and nationalists willing to stand for them.