"...colloquial term of Ostflucht - flight from the East.
This was not merely a pattern reflected in Germany, of course. The east-to-west migration pattern was common across Europe. Romanian and Galician minorities, in particular Jews, migrated to major cities like Budapest, Vienna, Pilsen and Prague. Anatolian Turks made their way to Salonica, Uskup, Sofia and Sarajevo. Italians were the most common migrant or seasonal worker in France, Finns had begun finding their ways to Swedish mines, logging camps and smelters by the late 1910s, and Spaniards who looked west only to the Atlantic found ample opportunities across the culturally similar Latin New World.
It held particular economic and political import within Germany, however, as it was a largely domestic and internal movement and that it had very particular ideological connotations for the Junkers. For decades, if not centuries, there had been a romanticized sense that Germany's destiny lay to its east, as it had in the Teutonic times, a belief as fundamental to then-contemporary German nationalism as the pacification of western North America had been in the national mythos of the United States and Canada. Not only was this a culturally expansionist policy, it was also one that was viewed as largely economically beneficial to the Junkers, who owned their vast estates east of the Elbe and due to their positions within the Prussian military and bureaucracy saw themselves as modern-day heirs to the Teutonic knights. Plans to colonize Polish-speaking territories of Prussia with German settlers had been government policy, explicitly, under Bismarck, and this stance was a fundamental component of the silent partnership between Germany and Russia to suppress Polish nationalism along their borderlands.
Something had happened, though, to prevent the Germanizing dreams of the Junkers in the East - the successful industrialization and urbanization of Germany, and the economic deadweight that the inefficient East Elbian estates represented within that paradigm. Between the final unification of Germany in 1868 and the start of the Central European War a half century later, tens of thousands of Germans from Posen, Pomerania and West Prussia in particular made their way westwards. Many wound up in the mines of Silesia, not too far from their ancestral homes, while most found their way to factories within Prussia throughout the booming Rhineland. They were followed close behind by many Poles, who within a generation despite worshipping in separate parishes and often attending separate schools were often in Rhenish cities nearly fully Germanized, even with surnames like Wisniewski or Kasinski.
The East only avoided stagnant population growth thanks to high birthrates and a different Ostflucht phenomenon, that of Russian Poles and Ashkenazi Jews from the Pale of Settlement shifting into the Posen region (and some Volga Germans emigrating from Russia, though in such small numbers as to have minimal impact). This ironically shifted the demographics of the population east of the Oder even less German, even as it drove down labor costs for itinerant farmers on the Junker estates. This helped take a bite out of the costs of the protective agricultural tariffs they continued to demand, but severely crippled the settlement policy in the East and created deep doubts in the Prussian state about its ability to successfully "Germanize" much of anything, and such views were before long staunchly held by the Kaiser - with major, considerable impacts down the line when it came time to draft the treaties that eventually ended the Central European War..."
- Heinrich: The Life and Legacy of Germany's Goldkaiser