Standing at the breach was one man: the Sixth Lubavitcher Rebbe, Rabbi Yosef Yitzchak Schneersohn (1880–1950), known among Chasidim as the Frierdiker Rebbe. His later successor and son-in-law, Rabbi Menachem Mendel Schneerson, would turn Chabad into a global force in the second half of the 20th century. But the foundations of that future were laid by Rabbi Yosef Yitzchak –charged by his father, the Fifth Rebbe, Rabbi Shalom Ber Schneersohn, to risk everything to preserve Jewish life under communism. Shortly before his passing in 1920, Rabbi Shalom Ber predicted the eventual disintegration of the Soviet Union, “a country dedicated to...