Left-wing anti-Semitism has its roots in the 19th century, writes Alan Johnson. In the 20th it was transformed, under Soviet auspices, into anti-Zionism. And now the anti-Zionist left is eager to find common cause with Islamist anti-Semites:

[L]eft-wing anti-Zionism has been converging with some forms of Arab nationalism and even political Islamism—which are both now coded as singularly progressive. The left has its own version of Orientalism, which infantilizes the Palestinians and Arabs, puts them beyond criticism, and makes them the subject of endless Western left-wing delusions. For example, take Jeremy Corbyn’s truly incredible claim that Hamas and Hizballah are “bringing about long-term peace and social justice and political justice in the whole region.”

This convergence . . . was smoothed by two developments on the left. In the East, the Communist bloc’s decades-long “anti-Zionist” propaganda campaign injected an “anti-imperialism of idiots” into the global left during the cold war. We are talking about the mass publication and global distribution of anti-Semitic materials through the Communist parties and their fellow travelers. . . . Two-hundred-and-thirty books were published in the USSR alone from 1969 to 1985 about a supposed Zionist-Masonic conspiracy against Russia. These books had a combined print run of 9.4 million.

In the West, . . . anti-imperialism . . . was raised to a radically new status in the 1960s. . . . [The Israeli-Palestinian conflict] was reframed. No longer was one people involved in a complex unresolved national question with another people. Now Israel became “a key site of the imperialist system” and the Palestinians became “the Resistance” to imperialism. . . .

Now, to support Israel’s enemies—whatever these enemies stood for, however they behaved—was a left-wing “anti-imperialist” duty: in other words, anti-Semitism went “progressive.”

Two extraordinary events demonstrate something important about Israel’s most fervent adversaries. One was a speech given at something called The People’s Forum (funded generously by Goldman Sachs), which stated, “When the state of Israel is finally destroyed and erased from history, that will be the single most important blow we can give to destroying capitalism and imperialism.”

The suggestion that this tiny state is the linchpin of a global, centuries-old phenomenon like capitalism goes well beyond anything resembling rational criticism. Even if Israel were guilty of genocide, apartheid, and oppression—which of course it is not—it would not follow that its destruction would help end capitalism or imperialism.

The other was an anti-Israel protest that took place in front of New York City’s Memorial Sloan Kettering Cancer Center, deemed “complicit” in Israel’s evils. At organizers’ urging, participants shouted their slogans at kids in the cancer ward, who were watching from the windows. Given Hamas’s indifference toward the lives of Gazan children, such callousness toward non-Palestinian children from Hamas’s Western allies shouldn’t be surprising. The protest—like the abovementioned speech—deliberately conveyed the message that Israel is the ultimate evil and its destruction the ultimate good, cancer patients be damned.

The fact that Israel’s adversaries are almost comically perverse does not mean that they can be dismissed. If its allies fail to understand the obsessive and irrational hatred that it faces, they cannot effectively help it defend itself.