With the dissolution of Austria–Hungary in 1918 according to the Treaty of Saint-Germain-en-Laye, both cities became part of the reconstituted Polish state, although the majority of the population was German, forming a German language island.

Some ethnic German citizens formed an anti-Polish, anti-Jewish Jungdeutsche Partei, supported financially by the Foreign Ministry of Nazi Germany.
Its members smuggled weapons and waged a campaign of intimidating other German residents to leave for Germany.
A considerable number of young Germans joined  this Party during the mid-1930s.

During World War II Bielsko-Biała was annexed by Nazi Germany.
Many of its Jewish Jewish residents were murdered at the nearby Auschwitz extermination camp.
After the defeat of Nazi Germany in 1945, the remaining German population fled westward or were expelled.
The town was polonized and gradually repopulated by Polish settlers.

Several widely known Holocaust survivors from Bielsko-Biała were Roman Frister, Gerda Weissmann Klein and Kitty Hart-Moxon, all of whom wrote accounts of their experiences during World War II.

The combined city of Bielsko-Biała was created administratively on 1 January 1951 when the two cities of Bielsko, and Biała (known until 1951 as Biała Krakowska), were unified.
With the dissolution of Austria–Hungary in 1918 according to the Treaty of Saint-Germain-en-Laye, both cities became part of the reconstituted Polish state, although the majority of the population was German, forming a German language island. 
Some ethnic German citizens formed an anti-Polish, anti-Jewish Jungdeutsche Partei, supported financially by the Foreign Ministry of Nazi Germany. Its members smuggled weapons and waged a campaign of intimidating other German residents to leave for Germany. A considerable number of young Germans joined  this Party during the mid-1930s. 
During World War II Bielsko-Biała was annexed by Nazi Germany. Many of its Jewish Jewish residents were murdered at the nearby Auschwitz extermination camp. After the defeat of Nazi Germany in 1945, the remaining German population fled westward or were expelled. The town was polonized and gradually repopulated by Polish settlers.
Several widely known Holocaust survivors from Bielsko-Biała were Roman Frister, Gerda Weissmann Klein and Kitty Hart-Moxon, all of whom wrote accounts of their experiences during World War II.
The combined city of Bielsko-Biała was created administratively on 1 January 1951 when the two cities of Bielsko, and Biała (known until 1951 as Biała Krakowska), were unified.
Bielsko-Biała