Complex sentence: But Vilnius/Vilna-before the German invasion-was a predominately Jewish and Polish city-after it the Jewish society and community to which Schwartzberg previously belonged did not exist anymore.
Split into simpler sentences: But Vilnius/Vilna-before the German invasion-was a predominately Jewish and Polish city-but the German invasion decimated of it's Jewish population. The Jewish society and community to which Schwartzberg previously belonged did not exist anymore.