In January 2000, heads of state or senior representatives of 44 governments met in Stockholm to commit themselves to a continuing program of Holocaust remembrance and the fight against anti-Semitism. Barely two years later, synagogues and Jewish schools in France and Belgium were being firebombed, and Jews were being attacked in the streets.

The distinguished Chief Rabbi of France, Rabbi Joseph Sitruk, advised Jews not to wear yarmulkas in the street. The French Jewish intellectual Alain Finkielkraut wrote, 'The hearts of the Jews are heavy. For the first time since the war, they are afraid." Shmuel Trigano, professor of sociology at the University of Paris, openly questioned whether there was a future for Jews in France. Never again had become ever again.

Basing Jewish identity on memories of persecution is a mistake.

In February 2002 I gave my first speech on the new anti-Semitism. Never before had I spoken on the subject. I had grown up without a single experience of anti-Semitism. I believed, and still do, that the whole enterprise of basing Jewish identity on memories of persecution was a mistake.

The distinguished Holocaust historian Lucy Dawidowicz reached the same conclusion at the end of her life. She warned of the danger of a whole generation of children growing up knowing about the Greeks and how they lived, the Romans and how they lived, the Jews and how they died. I wrote Radical then, Radical now, specifically to focus Jewish identity away from death to life, suffering to celebration, grief to joy.

The return of anti-Semitism, after 60 years of Holocaust education, interfaith dialogue and antiracist legislation is a major event in the history of the world. Far-sighted historians like Bernard Lewis and Robert Wistrich had been sounding the warning since the 1980s. Already in the 1990s, Harvard literary scholar Ruth Wisse argued that antisemitism was the most successful ideology of the twentieth century. German fascism, she said, came and went. Soviet communism came and went. Anti-Semitism came and stayed.

It is wrong to exaggerate. We are not now where Jews were in the 1930s. Nor are Jews today what our ancestors were: defenseless, powerless and without a collective home. The State of Israel has transformed the situation for Jews everywhere. What is necessary now is simply to understand the situation and sound a warning. That is what Moses Hess did in 1862, Judah Leib Pinsker in 1882 and Theodor Herzl in 1896: 71, 51 and 37 years respectively before Hitler's rise to power. To understand is to begin to know how to respond, with open eyes and without fear.

Today's anti-Semitism is a new phenomenon, continuous with, yet significantly different from the past. To fathom the transformation, we must first define what anti-Semitism is. In the past Jews were hated because they were rich and because they were poor; because they were capitalists (Marx) and because they were communists (Hitler); because they kept to themselves and because they infiltrated everywhere; because they held tenaciously to a superstitious faith (Voltaire) and because they were rootless cosmopolitans who believed nothing (Stalin).

Anti-Semitism mutates, defeating the immune system set up by cultures to protect themselves against hatred.

Anti-Semitism is not an ideology, a coherent set of beliefs. It is, in fact, an endless stream of contradictions. The best way of understanding it is to see it as a virus. Viruses attack the human body, but the body itself has an immensely sophisticated defense, the human immune system.

How then do viruses survive and flourish? By mutating. Anti-Semitism mutates, and in so doing, defeats the immune systems set up by cultures to protect themselves against hatred. There have been three such mutations in the past two thousand years, and we are living through the fourth.

The first took place with the birth of Christianity. Before then there had been many Hellenistic writers who were hostile to Jews. But they were also dismissive of other non-Hellenistic peoples. The Greeks called them barbarians. There was nothing personal in their attacks on Jews. This was not anti-Semitism. It was xenophobia.

This changed with Christianity. As was later to happen with Islam, the founders of the new faith, largely based on Judaism itself, believed that Jews would join the new dispensation and were scandalized when they did not. Jews were held guilty of not recognizing – worst still, of being complicit in the death of – the messiah. A strand of Judeophobia entered Christianity in some of its earliest texts, and became a fully-fledged genre, the 'Adversos Judaeos' literature, in the days of the Church Fathers. From here on, Jews – not non-Christians in general – became the target of what Jules Isaac called the 'teaching of contempt'.

The second mutation began in 1096 when the Crusaders, on their way to conquer Jerusalem, stopped to massacre Jewish communities in Worms, Speyer and Mainz, the first major European pogrom. In 1144 in Norwich there was the first Blood Libel, a myth that still exists today in parts of the Middle East. Religious Judeophobia became demonic. Jews were no longer just the people who rejected Christianity. They began to be seen as a malevolent force, killing children, desecrating the host, poisoning wells and spreading the plague. There were forced conversions, inquisitions, burnings at the stake, staged public disputations, book burnings and expulsions. Europe had become a 'persecuting society'.

We can date the third mutation to 1879 when the German journalist Wilhelm Marr coined a new word: anti-Semitism. The fact that he needed to do so tells us that this was a new phenomenon. It emerged in an age of Enlightenment, the secular nation state, liberalism and emancipation. Religious prejudice was deemed to be a thing of the past. The new hatred had therefore to justify itself on quite different grounds, namely race.

This was a fateful development, because you can change your religion. You cannot change your race. Christians could work for the conversion of the Jews. Racists could only work for the extermination of the Jews. So the Holocaust was born. Sixty years after the word came the deed.

Unlike its predecessors, new anti-Semitism focuses not on Jews as a religion or race, but as a nation.

Today we are living through the fourth mutation. Unlike its predecessors, the new anti-Semitism focuses not on Judaism as a religion, nor on Jews as a race, but on Jews as a nation. It consists of three propositions. First, alone of the 192 nations making up the United Nations, Jews are not entitled to a state of their own. As Amos Oz noted: in the 1930s, anti-Semites declared, 'Jews to Palestine'. Today they shout, 'Jews out of Palestine'. He said: they don't want us to be there; they don't want us to be here; they don't want us to be.

The second is that Jews or the State of Israel (the terms are often used interchangeably) are responsible for the evils of the world, from AIDS to global warming. All the old anti-Semitic myths have been recycled, from the Blood Libel to the Protocols of the Elders of Zion, still a best-seller in many parts of the world. The third is that all Jews are Zionists and therefore legitimate objects of attack.

The bomb attacks on synagogues in Istanbul and Djerba, the arson attacks on Jewish schools in Europe, and the almost fatal stabbing of a young yeshiva student on a bus in North London in October 2000, were on Jewish targets, not Israeli ones. The new anti-Semitism is an attack on Jews as a nation seeking to exist as a nation like every other on the face of the earth, with rights of self-governance and self-defense.

How did it penetrate the most sophisticated immune system ever constructed – the entire panoply of international measures designed to ensure that nothing like the Holocaust would ever happen again, from the United Nations Universal Declaration of Human Rights (1948) to the Stockholm declaration of 2000? The answer lies in the mode of self-justification. Most people at most times feel a residual guilt at hating the innocent. Therefore anti-Semitism has always had to find legitimation in the most prestigious source of authority at any given time.

In the first centuries of the Common Era, and again in the Middle Ages, this was religion. That is why Judeophobia took the form of religious doctrine. In the nineteenth century, religion had lost prestige, and the supreme authority was now science. Racial anti-Semitism was duly based on two pseudo-sciences, social Darwinism (the idea that in society, as in nature, the strong survive by eliminating the weak) and the so-called scientific study of race.

By the late twentieth century, science had lost its prestige, having given us the power to destroy life on earth. Today the supreme source of legitimacy is human rights. That is why Jews (or the Jewish state) are accused of the five primal sins against human rights: racism, apartheid, ethnic cleansing, attempted genocide and crimes against humanity.

That is where we are. How then shall we respond? There are three key messages, the first to Jews, the second to anti-Semites, and the third to our fellow human beings in this tense and troubled age. As Jews we must understand that we cannot fight anti-Semitism alone. The victim cannot cure the crime. The hated cannot cure the hate. Jews cannot defeat anti-Semitism. Only the cultures that give rise to it can do so.

European Jews in the nineteenth and early twentieth century made one of the most tragic mistakes in history. They said: Jews cause anti-Semitism, therefore they can cure it. They did everything possible. They said, 'People hate us because we are different. So we will stop being different.' They gave up item after item of Judaism. They integrated, they assimilated, they married out, they hid their identity. This failed to diminish anti-Semitism by one iota. All it did was to debilitate and demoralize Jews.

We need allies. Jews have enemies but we also have friends and we must cultivate more. I have helped lead the fight against Islamophobia; I ask Muslims to fight Judeophobia. I will fight for the right of Christians throughout the world to live their faith without fear; but we need Christians to fight for the right of Jews to live their faith without fear.

The most important thing Jews can do to fight anti-Semitism is to never internalize it.

The most important thing Jews can do to fight anti-Semitism is never, ever to internalize it. That is what is wrong in making the history of persecution the basis of Jewish identity. For three thousand years Jews defined themselves as a people loved by God. Only in the nineteenth century did they begin to define themselves as the people hated by gentiles. There is no sane future along that road. The best psychological defense against anti-Semitism is the saying of Rav Nachman of Bratslav: 'The whole world is a very narrow bridge; the main thing is never to be afraid.'

To anti-Semites and their fellow travelers we must be candid. Hate destroys the hated, but it also destroys the hater. It is no accident that anti-Semitism is the weapon of choice of tyrants and totalitarian regimes. It deflects internal criticism away by projecting it onto an external scapegoat. It is deployed in country after country to direct attention away from real internal problems of poverty, unemployment and underachievement. Anti-Semitism is used to sustain regimes without human rights, the rule of law, an independent judiciary, a free press, liberty of association or accountable government. One truth resounds through the pages of history: To be free you have to let go of hate. Those driven by hate are enemies of freedom. There is no exception.

Finally to all of us together, we must say: Jews have been hated throughout history because they were different. To be sure, everyone is different; but Jews more than most fought for the right to be different. Under a succession of empires, and centuries of dispersion, Jews were the only people who for more than two thousand years refused to convert to the dominant religion or assimilate into the dominant culture. That is why anti-Semitism is a threat not just to Jews but to humanity.

God, said the rabbis, makes everyone in His image, yet He makes everyone different to teach us to respect difference. And since difference is constitutive of humanity, a world that has no space for difference has no space for humanity. That is why a resurgence of anti-Semitism has always been an early warning of an assault on freedom itself. It is so today.

We must find allies in the fight against hate. For though it begins with Jews, ultimately it threatens us all.

This article first appeared in the Jewish Chronicle.