Ever since there have been loyalty tests and demands for left-wing Jews to speak "as a Jew" as Howard Jacobson puts it, and announce their shame of and contempt for Israel. The campaign to boycott Israeli universities, to quote the most sinister example, says that Israeli academics must prove that they are not anti-Palestinian before it will allow them to work with the Europeans — in the same manner as McCarthyites insisted that Hanns Eisler prove that he was not anti-American before they would allow him to work in the US.

Finally, who can deny that anti-Semitism was popular in Eastern Europe and Russia then, and is popular in the Middle East and parts of Europe now? Particularly if the more obviously "medieval" features of the prejudice are dropped and anti-Semitism is recast as anti-Zionism.

Leftist attitudes have been extraordinarily consistent. Communism gave way to anti-colonialism. Israel remained a target for special rage, even though Zionism was both a settler movement and an anti-colonial movement that attacked the forces of the British empire. Anti-colonialism gave way to the 1968 rebellions. In Germany, the left-wing terrorists around the Baader Meinhof gang claimed to be against the Nazism of their parents. They proved they were just like their parents when they planted a bomb at a meeting to commemorate the anniversary of Kristallnacht. Today the bulk of the Left has forgotten its old appeals to universal values. Conservatives and liberals are more likely to emphasise the need to assimilate, while the Left "celebrates" diversity and difference — for everyone, that is, except Jews.

I could go on but must anticipate an objection. Sensible and well-meaning people might say, "If hatred of Israel from the Left has been remarkably consistent, that's the fault of Israel and those Jews who support it." They have a small point.

Israel is unique among nations. It is the only sovereign country whose right to exist is questioned as a matter of routine. No other country — not China, Zimbabwe, Sudan or Iran — suffers comparable abuse. Left-wing viewers of the atrocities in Syria must be baffled. Who knew that the Assad family was capable of such crimes? Who told them that anyone in the Middle East outside Palestine had good cause to revolt?

But those who say that the bias of much leftish protest is a reason to exonerate Israel miss the point that injustice in Palestine is still injustice, and people are entitled to protest against it. Anti-Semitism plays a part in Israel-hating, so does the unhappy status of Jerusalem as a sacred city for fanatics from three warring creeds. But many who defend the rights of Palestinians are not fanatics. They may devote disproportionate energy to their cause. They may or may not be blind to the suffering of others. But that is no reason to damn them. They oppose Israeli policy because contrary to the Balfour Declaration it impinges on the civil rights of non-Jewish communities.