Due losing the First World War in October 1918 the “multinational” Austro-Hungarian Monarchy collapsed, giving Poles and Czechs the chance to live in their own nation states. The National Council for the Duchy of Cieszyn came into being as early as the middle of October, and assumed power over most of Cieszyn Silesia. Its headquarters were in the castle while the German-dominated municipal authorities, that did not recognize the National Council, continued to be based in the town hall. The Czechoslovakian authorities in Prague also did not recognize the competence of the Council and the victorious powers were engaged in a dispute over the territory of Cieszyn Silesia. The powers were represented by two international committees (a mediatory one in 1919 and a plebiscitary one in 1920) operating out of the Pod Brunatnym Jeleniem Hotel. Finally, in July 1920 Cieszyn Silesia – and Cieszyn itself – was divided up between Poland and Czechoslovakia. The part on the right bank, along with the historical centre, was granted to Poland. The suburbs on the left bank, which would become the new town of Český Těąín, were granted to Czechoslovakia.
       A great many Austrian government officials left the Polish part of Cieszyn after the First World War, others preferred to move to the Czechoslovakian part of the town. Their places were taken by newcomers from Poland which resulted in the fact that in 1921 only 30% of the residents were self-declared German speakers. In 1921 Cieszyn had a population of over 15,000 people; ten years later – 18,000. It initially occupied 424 ha, and then expanded due to the incorporation of the neighbouring villages; Błogocice in 1922 and Bobrek in 1932. 
       A mixed Polish-German Administrative Committee led by the Polish police commissioner ran the municipal authorities between 1920 and 1922. In 1922 Cieszyn and its country was included in the newly created Silesian Province and the elected Town Council held office. Poles, now the town’s prevailing majority, dominated the council, but the members of the German minority still played a significant role; one of the deputy mayor’s positions was reserved for them. One of the first changes made by the municipal authorities was to remove German language signs and notices left over from former times. Monuments to the Emperor were taken down from squares and streets were given Polish names – mostly traditional ones. The situation changed after 1935 during the rule of Sanacja when everything that did not sound Polish enough was polonized by order. The local authority was led by Jan Michejda (1922-1927), Fr. Józef Londzin (1927-1929), Władysław Michejda (1929-1937) and Rudolf Halfar (1937-1939). Poles and Germans were capable of working together harmoniously for the good of the town – now finding itself in straightened circumstances – as were Jews, who made up approximately 10% of the population.
       In fact, no-one was spared effects of the division of the hitherto homogenous town. Almost every family ended up with relatives on the other side of the border, which was difficult to cross during the interwar period. Cieszyn was cut off from its former infrastructure, and earlier economic ties were broken, leading to considerable economic recession which was intensified by the worldwide crisis of the 1920s. Several years passed before an infrastructure independent of Český Těąín was built. However, Polish institutions and schools were rapidly founded. The activities of the German schools were first curtailed and then the schools were closed. In their place a Polish state teaching college and two gymnasiums were founded, to say nothing of several elementary and vocational schools. The difficult situation of Cieszyn was alleviated by the fact that almost all the formerly jointly used buildings were left on the Polish side. The only significant institution erected at that time was the out-patient clinic in ul. Bielska. Cieszyn started to develop again in the late 1930s but out of 11 factories founded between 1921 and 1936 only 3 were large i.e. with workforces of between 200 and 400. Cieszyn received a new connection with Upper Silesia in 1934 with the construction of a railway line to Zebrzydowice, but a water supply system was only built in 1937. In addition all the museum collections existing in the town were brought together in a central museum, whose collections were placed in Cieszyn castle, the best location for this purpose, in 1922. However, in 1929 the museum had to leave these rented premises and eventually moved to the mansion in ul. Regera that the town had purchased from the Demel family (the former Larisch family residence). 
       The other part of Cieszyn, formerly the industrial suburbs of the town, experienced much more substantial changes. Among the more important buildings situated here were only the railway station, St. Mary’s Catholic Church, two German schools and a few large factories. In 1921 Český Těąín had a population of over 8,000 but only 399 houses. The town was short of public buildings which had to be built from scratch. The Government Committee initially managing the town obtained a low-interest state loan of 15,000,000 crowns that were allotted for the erection of schools and other communal buildings. The project was continued by the Town Council, appointed in 1923, which elected Józef Kożdoń, the founder and leader of Silesian People’s Party (the so–called ślązakowcy), as mayor. In 1921 only 30% inhabitants declared themselves as Czechoslovakians, 50.6% Germans, 10% Jews and 8.2% Poles. Over 8,000 people inhabited the town at that time; in 1931 the number of residents of Český Těąín increased to 10,500 out of which most were Czechs (up to 45%) and Poles (up to 15.5%) while the number of Germans had decreased. Still, German parties and the ślązakowcy constituted the majority in communal elections due to which Kożdoń remained mayor until 1938. 
       In spite of political and national differences, the local authority under Kożdoń’s leadership erected all essential municipal buildings in less than 20 years; the Starostwo in 1924-27, the cemetery in 1924-25, the abattoir in 1926-29, the post office in 1932, the building of the county health service in 1931. One of the last touches was the building of the new town hall in Český Těąín, completed in 1929, and the surrounding town square. A great deal of effort and funds were put into founding new schools especially for Czech and Polish children. The most magnificent school building in Český Těąín – the Czech gymnasium – was completed in 1935, and the hospital in 1937.
       Churches, other institutions and private investors all contributed to the considerable number of new buildings going up. A Catholic parish, with the Church of St. Mary, grew quite quickly. The Lutheran church was divided into two congregations; the German congregation built a church in námĕstí Dr. Martina Luthra, and the Polish one in Na Nivách. A separate place of worship was built by the Church of the Czech Brethren. The Jewish Community, which had also been divided, created its own community in 1923. In the next few years a Jewish Community synagogue was built, also an orthodox synagogue, and two private houses of prayer. The number of houses in Český Těąín between 1924 and 1938 doubled, reaching 800, and whole new districts came into life; for example Na Rozvoji. 
       In other words, in the space of over ten years Český Těąín was transformed into a town with full civic rights with all essential institutions, buildings and infrastructure. Český Těąín had became the cultural focus for the region, carrying out the same functions as the whole town of Cieszyn had done previously. Český Těąín was also an important centre for the Polish minority due to its central location and transport links. Polish schools and the most important Polish organizations such as Macierz Szkolna w Czechosłowacji and Rodzina Opiekuńcza were based there. The town’s economic base was industry and small-scale crafts, while, of course, the building trade prospered. A great deal of people worked also in trade, finances, the professions and transportation. Similarly to Cieszyn over 150 different clubs and associations were active in Český Těąín. 
       After the annexation of so-called Zaolzie into Poland in October 1938 both parts of the town were united and the municipal authorities chose the town hall in Český Těąín (at that time called Cieszyn Zachodni – West Cieszyn) as their headquarters. The Polish authorities only recognized as citizens those people who had lived there before 1918. As a result many local people, even those who had been born there, had to leave the town. Czech schools became Polish ones. 
       The town remained united during the Second World War when Cieszyn was incorporated into the Third Reich. When the Nazis took control of the town without encountering resistance, the deputy mayor, Artur Gabrisch, became the acting mayor. He was replaced by Wilhelm Koperberg after only a month and newcomers from the Reich appropriated most of the town’s official positions in the town and the newly created district of Teschen. Polish activists were removed or detained and the use of Polish in public was forbidden. The Polish and Czech populations were subject to multiple forms of discrimination; many were publicly executed, for example in Pod Walką in 1942. The Nazis established a prisoner of war camp – Stalag VII – in Konteąinec in the former First World War barracks. The fate of the Jewish community was even direr, as they were forced to flee or transported to concentration camps. The Jewish synagogues and the houses of prayer were destroyed in both parts of Cieszyn, with the exception of the Schomre Schabos synagogue in ul. J. Bożka. Not much came of German plans to make Cieszyn a huge economic and cultural centre in the southern part of Upper Silesia. After the war the two towns returned to their status of 1937 .

J. Spyra, transl. D. French