On 8 November 1918, at the end of World War I, a socialist workers' council forced Duke Ernest Augustus to abdicate.
On 10 November, the council proclaimed the Socialist Republic of Brunswick under one-party government by the Independent Social Democratic Party of Germany (USPD); however, the subsequent Landtag election on 22 December 1918 was won by the Social Democratic Party of Germany (MSPD), and the USPD and MSPD formed a coalition government.
An uprising in Braunschweig in 1919, led by the communist Spartacus League, was defeated when Freikorps troops under Georg Ludwig Rudolf Maercker took over the city on order of the German Minister of Defence, Gustav Noske.
An SPD-led government was subsequently established; in December 1921, a new constitution was approved for the Free State of Brunswick, now a parliamentary republic within the Weimar Republic, again with Braunschweig as its capital.

After the Landtag election of 1930, Brunswick became the second state in Germany where the Nazis participated in government, when the National Socialist German Workers' Party (NSDAP) formed a coalition government with several conservative and right-wing parties.
With the support of Dietrich Klagges, Brunswick's minister of the interior, the NSDAP organized a large SA rally in Braunschweig.
On 17–18 October 1931, 100,000 SA stormtroopers marched through the city, street fights between Nazis, socialists, and communists left several dead or injured.
On 25 February 1932, the state of Brunswick granted Adolf Hitler German citizenship to allow him to run in the 1932 German presidential election.
In Braunschweig, Nazis carried out several attacks on political enemies, with the acquiescence of the state government.

After the Nazi seizure of power in 1933, several state institutions were placed in Braunschweig, including the Luftfahrtforschungsanstalt in Völkenrode, the Hitler Youth Academy for Youth Leadership, and the SS-Junkerschule Braunschweig.
With the Reichswerke Hermann Göring in Salzgitter and the Stadt des KdF-Wagens, as well as several factories in the city itself (including Büssing and the Volkswagenwerk Braunschweig), the Braunschweig region became one of the centres of the German arms industry.

During the Second World War, Braunschweig was an Untergebiet Hauptquartier ("Sub-Area Headquarters") of Wehrkreis XI ("Military District XI"), and was the garrison city of the 31st Infantry Division that took part in the invasions of Poland, Belgium, and France, largely being destroyed during its retreat following the invasion of Russia.
In this period, thousands of Eastern workers were brought to the city as forced labor, and in the 1943–1945 period at least 360 children taken away from such workers died in the Entbindungsheim für Ostarbeiterinnen ("Maternity Ward for Eastern Workers").

In 1944, a subcamp of the concentration camp Neuengamme was established in Braunschweig.
 Hundreds of prisoners, mostly Jews, lived in brutal conditions and hundreds died from hunger, disease, and overwork.
 Piera Sonnino, an Italian author, writes of her imprisonment in that camp in her book, This Has Happened, published in English in 2006 by MacMillan Palgrave.

The Allied air raid on October 15, 1944, destroyed most of the city's churches, and the Altstadt (old town), the largest homogeneous ensemble of half-timbered houses in Germany.
The city's cathedral, which had been converted to a Nationale Weihestätte (national shrine) by the Nazi government, still stood.
On 8 November 1918, at the end of World War I, a socialist workers' council forced Duke Ernest Augustus to abdicate. On 10 November, the council proclaimed the Socialist Republic of Brunswick under one-party government by the Independent Social Democratic Party of Germany (USPD); however, the subsequent Landtag election on 22 December 1918 was won by the Social Democratic Party of Germany (MSPD), and the USPD and MSPD formed a coalition government. An uprising in Braunschweig in 1919, led by the communist Spartacus League, was defeated when Freikorps troops under Georg Ludwig Rudolf Maercker took over the city on order of the German Minister of Defence, Gustav Noske. An SPD-led government was subsequently established; in December 1921, a new constitution was approved for the Free State of Brunswick, now a parliamentary republic within the Weimar Republic, again with Braunschweig as its capital.
After the Landtag election of 1930, Brunswick became the second state in Germany where the Nazis participated in government, when the National Socialist German Workers' Party (NSDAP) formed a coalition government with several conservative and right-wing parties. With the support of Dietrich Klagges, Brunswick's minister of the interior, the NSDAP organized a large SA rally in Braunschweig. On 17–18 October 1931, 100,000 SA stormtroopers marched through the city, street fights between Nazis, socialists, and communists left several dead or injured. On 25 February 1932, the state of Brunswick granted Adolf Hitler German citizenship to allow him to run in the 1932 German presidential election. In Braunschweig, Nazis carried out several attacks on political enemies, with the acquiescence of the state government.
After the Nazi seizure of power in 1933, several state institutions were placed in Braunschweig, including the Luftfahrtforschungsanstalt in Völkenrode, the Hitler Youth Academy for Youth Leadership, and the SS-Junkerschule Braunschweig. With the Reichswerke Hermann Göring in Salzgitter and the Stadt des KdF-Wagens, as well as several factories in the city itself (including Büssing and the Volkswagenwerk Braunschweig), the Braunschweig region became one of the centres of the German arms industry.
During the Second World War, Braunschweig was an Untergebiet Hauptquartier ("Sub-Area Headquarters") of Wehrkreis XI ("Military District XI"), and was the garrison city of the 31st Infantry Division that took part in the invasions of Poland, Belgium, and France, largely being destroyed during its retreat following the invasion of Russia. In this period, thousands of Eastern workers were brought to the city as forced labor, and in the 1943–1945 period at least 360 children taken away from such workers died in the Entbindungsheim für Ostarbeiterinnen ("Maternity Ward for Eastern Workers").
In 1944, a subcamp of the concentration camp Neuengamme was established in Braunschweig.  Hundreds of prisoners, mostly Jews, lived in brutal conditions and hundreds died from hunger, disease, and overwork.  Piera Sonnino, an Italian author, writes of her imprisonment in that camp in her book, This Has Happened, published in English in 2006 by MacMillan Palgrave.
The Allied air raid on October 15, 1944, destroyed most of the city's churches, and the Altstadt (old town), the largest homogeneous ensemble of half-timbered houses in Germany. The city's cathedral, which had been converted to a Nationale Weihestätte (national shrine) by the Nazi government, still stood.
Braunschweig