“I am convinced that there are only a few people in this hall who will not experience the great day.” August Bebel had plenty of swagger in 1891 — and he wasn’t alone. As he spoke, Rosa Luxemburg recounted, “a warm, electric stream of life, of idealism, of security in joyful action” swept through the crowd.

The Second International was just two years old and now, at the pivotal Erfurt Congress, the German Social Democrats — the largest socialist party in the world — were laying the groundwork for generations of working-class politics.

In the years that followed, socialists had plenty of cause for optimism. In election after election, labor and social-democratic parties saw their vote totals climb thanks to newly enfranchised workers. It seemed natural — both to terrified capitalists and ambitious trade unionists — that working-class political rights were translating into broader social emancipation.