The two prominent social democratic thinkers and close friends, the orthodox Marxist Karl Kautsky and the revisionist Eduard Bernstein, were completely out of contact due to their dispute in the revisionism debate between the turn of the century and 1912. Nevertheless, during these years, the two engaged in another debate, primarily implicitly: the question of Jewish nationality and Zionism. Yuval Rubovitch's book reveals for the first time how Bernstein and Kautsky developed their opposing positions on this topic and influenced each other, even though Bernstein was still seemingly a staunch anti-Zionist at that time. This intellectual debate extended until Bernstein's death—as a pro-Zionist—on the eve of National Socialism and was closely linked to the economic discourse of the two theorists.