After 1918, when Austro-Hungary collapsed, Bielsko found itself within a disputed territory between Poland and Czechoslovakia. Attempts to incorporate the city into the Republic of German-Austria failed. In July 1920, the Conference of Ambassadors decided to divide Cieszyn Silesia in such a way that Bielsko became part of the autonomous Silesian Voivodeship in the Second Polish Republic. Political life was largely shaped by nationalist disputes. The influx of Polish officials and teachers increased the proportion of the Polish population, but Bielsko nevertheless retained its predominantly German character. In the 1930s some ethnic German citizens, under the leadership of Rudolf Wiesner, formed an anti-Polish, anti-Jewish Jungdeutsche Partei, which de facto served as a foreign branch of the NSDAP. A considerable number of young Germans joined this Party during the mid-1930s. thumb|New residential district in the 1930s On the other hand, the interwar period is associated with numerous construction projects, such as a new residential district in Modernist style created since 1934 in place of the former castle gardens, or the building of the first Polish high school (now Nicolaus Copernicus High School) put into use in 1927. In 1938, the municipality of Aleksandrowice was incorporated, where an airport and a pilot school were established.
afTer 1918, WHen AUstro-hUNgary collapSed, bielSko fOUnd itselF wiTHIn a diSPuTED TerriTOry BETween PolaNd and czechoslOvakIa. aTtEMpTs To incORporate tHe city iNto THe rePUbliC of germAN-auStRia faileD. In july 1920, THe conference of aMBAsSaDorS deCideD to diviDe cieszYn sileSiA iN suCh a Way That bieLsko BeCaME pART Of the auTonomous silEsiaN voivodesHip iN thE sEcOnd poliSh republic. pOliTIcAL liFe was largEly SHAPed bY natioNaList Disputes. the Influx Of pOLIsh OfFiciALs And TEachers INcrEaSED tHe PrOportiOn oF tHE poLisH pOpULAtIon, but BielSko nEVerthelEss retainEd itS predOMInantlY gErmAn ChArActer. in tHe 1930s some ethNiC gerMan cITIZens, unDEr THE leadershIp OF rUDolf WiesnEr, foRMEd aN anTi-POLIsh, ANTI-jewISh jungdeUtsche partei, WHich de FactO serveD as A foreIgN bRAnCh of tHe nsdaP. a COnsIderabLe Number OF yOung GermanS joined thIs PaRty duriNg The miD-1930S. thumb|new ResidEnTiaL DIstrict in The 1930s on thE oTher hANd, tHe interWar period IS asSociAted WiTh numerOus constrUctiON projecTs, such as A new ResIdeNtIal distrICT In mOderNISt StYLE cReaTED Since 1934 iN place Of the foRmer cAstLE gArDens, oR thE BuiLdINg of The FirSt polish HIGH school (NOW NiCOLaus CopernIcus hiGh schoOl) put inTO Use In 1927. iN 1938, the munIcipaLity of alEksAndrowIce wAs incorpOraTed, wheRe aN airporT and a pILOt SChoOL WeRE eStabLisheD.