The durability of anti-Semitism is a horrifying marvel of history. Sara Bloomfield, the director of the Holocaust Museum, observes: "Anti-Semitism has existed with and without Christianity. With and without the right wing. With and without the left wing. With and without democracy. With and without economic problems. With and without globalization. With and without a Jewish homeland."

Why the Jews? It is a question that must often have been asked during pogroms and in ghettos and in prison camps. There are many answers, and none of them adequate. Anti-Semitism in the West has undeniable theological roots -- the distortion of a faith, founded by a Jewish teacher, to justify the persecution of Jews. Anti-Semitism has been fed by government incitement and by blood libels that never seem to die. It found resonance in various forms of nationalism and nativism, in the bent science of eugenics, and eventually in totalitarian ideology.

David Berger, the editor of "History and Hate," writes, "We shall never fully understand anti-Semitism. Deep-rooted, complex, endlessly persistent, constantly changing yet remaining the same, it is a phenomenon that stands at the intersection of history, sociology, economics, political science, religion and psychology."

But we do know that anti-Semitism has always been a kind of test -- a reliable measure of a nation's moral and social health. When the rights of Jews are violated, all human rights are insecure. When Jews and Jewish institutions are targeted, all minorities have reason for fear. And by this standard, America has cause for introspection.

The museum that von Brunn assaulted is the best answer to his hatred -- the aging survivors who still volunteer, photographs revealing the vanished lives of the dead, the happy pictures drawn by murdered children. Not far from where von Brunn entered the museum, there is a black wall inscribed with a quotation: "All men are created equal ... they are endowed by their creator with certain inalienable rights ... among these are life, liberty and the pursuit of happiness."

This is what anti-Semitism ultimately must deny; and this is the reason anti-Semitism must always and everywhere be confronted.