The Holocaust devastated European Jewry. The most strictly religious among them — the mystical-oriented Hasidic followers of historic rabbinic lineages and the mitnagdim, Hasidism’s more intellectually focused religious critics — suffered some of the worst losses.
Their insularity and suspicion of the larger world served them poorly at a time when maneuverability and adaptability might have helped them flee Nazi Europe for safety. Instead, they turned their noses up at non-Orthodox Jews and avoided dealing with non-Jews as much as possible.
This was true for both Hasidic and mitnagdim Jews, who are often lumped together by outsiders under the rubric “ultra-Orthodox.”
It’s a label many of them reject; they argue there’s nothing “ultra” about them and that they’re only adhering closely to what they think of as “normative” rabbinic Judaism.
In Hebrew, they’re called Haredi or Haredim, the plural. That’s how I’ll refer to them in this post.
Samuel Heilman, an American academic expert on Haredi life, wrote the following on the subject for a PBS show on Hasidic Jews.
The three things the rebbes told their Hasidim to do led to their being blown away. The rebbes said: "Don't go to America, the treyfe medina (the unclean country), and don't go to the Zionist state, Palestine. Don't change your clothes or learn the surrounding language." So they couldn't disguise themselves or pass as gentiles. And, the rebbes said, "Stay close to me." They did stay close to the rebbes, but many of the rebbes [the Belzer, the Satmar, the Gerer] ran off and left all their people to die.
David Ben-Gurion, the secular Jewish Zionist leader who was Israel’s first prime minister, was convinced that circumstances following World War II would further depress Haredi numbers. Back then, the Haredim comprised just 5 percent of Mandatory Palestine’s pre-state Jewish population.
However to gain United Nations backing for an independent Jewish state, Ben-Gurion believed he had to show full Jewish unity for such a move. That included Haredi support.