This book discusses the historical record of the idea that language is associated with national identity, demonstrating that different applications of this idea have consistently produced certain types of results. Nationalist movements aimed at ‘unification’, based upon languages which vary greatly at the spoken level, e.g. German, Italian, Pan-Turkish and Arabic, have been associated with aggression, fascism and genocide, while those based upon relatively homogeneous spoken languages, e.g. Czech, Norwegian and Ukrainian, have resulted in national liberation and international stability. It is also shown that religion can be more important to national identity than language, but only for religious groups which were understood in premodern times to be national rather than universal or doctrinal, e.g. Jews, Armenians, Maronites, Serbs, Dutch and English; this is demonstrated with discussions of the Holocaust, the Armenian Genocide, the civil war in Lebanon and the breakup of Yugoslavia, the United Netherlands and the United Kingdom.