The supposition that there would be a gap between the civilisations of the German west and the Polish (or Slavic) east, which helped legitimise the Prussian domination in the 19th century, should be challenged. The Prussian province of Posnania highlights the contradictions that could sometimes exist between different national views. Indeed, at the time of social and economic achievements by the local Polish minority, the negative image of “Polish anarchy” and the need for “German culture” could seem to be refuted. This essay attempts to show how the idea of a social frontier eventually failed to strengthen the German population’s identification with its eastern “Heimat”-lands. Thomas Serrier is maître de conférences for German history (19th-20th century) at the Institut d’études européennes (Université Paris VIII). He is the author of Entre Allemagne et Pologne. Nations et identités frontalières 1848-1914, Paris, Belin, 2002 and Günter Grass. Tambour battant contre l’oubli, Paris, Belin, 2004.