In the medieval period, Christianity united Europeans across language and culture—but then the Reformation fractured the “universal” Catholic Church and the Enlightenment undermined religion’s hold on the collective imagination. Nationalism emerged to fill the spiritual void with an idea of community based (loosely) on shared language and ethnicity. By the 19th century Europeans took it for granted each nation had a distinct “character” and inhabited sacred, inviolable territory. So when Germany annexed Alsace-Lorraine in 1871, it injured French national pride and provoked “revanchism” (desire for revenge). At the same time, nationalism threatened Austria-Hungary, an old-fashioned medieval empire with a dozen nationalities who wanted out.