It began with Israel’s exclusion from the ranks of the nonaligned nations more than 50 years ago, when Arab states refused to attend a 1955 nonaligned conference in Indonesia if an Israeli delegate was present. The Jewish state was snubbed in favor of such feudal kingdoms as Saudi Arabia, Libya and Yemen. And Israel’s collusion with imperial powers like Britain and France during the Suez crisis the following year cemented its ostracism.

Given the deep remorse for the misdeeds of colonialism, it was easier for the New Left of the 1960s to identify with the emerging Palestinian national movement than with the already established social democratic Israel. This deepening hostility toward Israel was present in Europe before the 1967 Arab-Israeli war and before the rush to build settlements on the West Bank.

AMID this rising hostility toward Israel, the French philosopher and political activist Jean-Paul Sartre advocated a different way forward. He was scarred by the memory of what had happened to France’s Jews during World War II — the discrimination, betrayals, deportations and exterminations. He understood the legitimacy of Israel’s war for independence and later commented that the establishment of the state of Israel was one of the few events “that allows us to preserve hope.” Yet Sartre also strongly supported Algeria’s fight for independence from France.

This double legacy of supporting Israel and the Algerian struggle symbolized the predicament of the entire postwar European left. Sartre argued that the left shouldn’t choose between two moral causes and that it was up to the Jews and the Arabs to resolve their conflict through discussion and negotiation. Sartre tried to create a space for a dialogue, lending his name and prestige to private and public meetings between the two sides such as the Comité Israël-Palestine in the 1970s. His approach reached its apogee with the many quiet meetings between Israelis and Palestinians in Europe that eventually led to the Oslo accords. But Sartre’s vision was stymied as Israeli settlements proliferated after 1977, strengthening the left’s caricature of Israel as an imperialist power and a settler-colonial enterprise. Some prominent voices on the European left have mouthed time-honored anti-Semitic tropes in their desire to appear supportive of the Palestinian cause. Ken Livingstone, a former newspaper editor and mayor of London, has a long history of insensitive remarks about Jews — from publishing a cartoon in 1982 of Menachem Begin, then Israel’s prime minister, in Gestapo uniform atop a pile of Palestinian skulls to likening a known Jewish reporter to “a concentration camp guard” 20 years later. Today, he contributes to Press TV, the English-language outlet for the Iranian government.

Sometimes the left distinguishes between vulnerable European Jews who have been persecuted and latter-day “Prussians” in Israel. Yet it is often forgotten that a majority of Israelis just happen to be Jews, who fear therefore that what begins with the delegitimization of the state will end with the delegitimization of the people.

Such Israelophobia, enunciated by sections of the European left, dovetailed neatly with the rise of Islamism among Palestinians and throughout the Arab world. The Islamist obfuscation of “the Jew” mirrored the blindness of many a European Marxist. Despite the well-intentioned efforts of many Jews and Muslims to put aside their differing perspectives on the Israeli-Palestinian conflict, the offensive imagery of “the Jew” has persisted in many immigrant communities in Western Europe. Islamists were willing to share platforms with socialists and atheists, but not with Zionists.

The New Left’s profound opposition to American power, and the convergence of reactionary Islamists and unquestioning leftists was reflected in the million-strong London protest against the invasion of Iraq in 2003. It was organized by the Muslim Association of Britain, the Trotskyist Socialist Workers Party and the Stalinist Communist Party of Britain. When some Muslims voiced apprehension about participating in the protest with non-Muslims, the M.A.B. leadership decreed that it was religiously permissible if halal food was provided and men and women were given separate areas. Such displays of “reactionary clericalism,” as the early Bolsheviks would have called it, were happily glossed over.