In the days of the Second Reich, Danzig, Memel, and Königsberg were the easternmost enclaves of Germandom. The German states of these cities were threatened, not only geopolitically by the Russian Empire that nearly surrounded them, but demographically and ethnolinguistically by the relatively more fecund Poles, Lithuanians, and kindred peoples. The Ostflucht, the migration of ethnic Germans from the eastern reaches of the Prussian realm to richer areas in western and central Germany, began almost as soon as Germany was unified. The recreation of independent Polish and Lithuanian nation-states impinged directly upon East Prussia, which became a sovereign German island in a Balto-Slavic sea. Unsurprisingly, the failure of Nazi Germany's gambit to unite all Germans into a single nightmarish empire left East Prussia and its adjoining cities forfeit. Memel was renamed Klaipeda and restored to a post-war Lithuania now a Soviet republic; Danzig became Gdansk as part of its annexation to a Poland that now also included the Warminsko-Mazurskie Voivodship, the south of East Prussia; the core of East Prussia, around the devastated city of Königsberg, became the Russian republic's 'Калининград province (Kaliningrad in Latin script).
Hermann's survey of these three cities, though six years out of date. Gdansk's Poland and Klaipeda's Lithuania are rejoining their old circum-Baltic and European communities, and using the remaining continuities with the pre-1939 past as capital to support this reintegration. The city of Kaliningrad and its province, trapped by their peripheral geography within the Russian Federation, can't follow suit. It's likely going too far to claim that Kaliningrad could become a "black hole" inside the European Union, despite the province's serious serious HIV/AIDS epidemic and the growing gap opening between it and its neighbours. It does seem quite likely that Kaliningrad won't be able to capitalize as effectively on its East Prussian heritage as its neighbour cities.