Theresienstadt

The Theresienstadt concentration camp was established by the SS during World War II in the fortress and garrison city of Terezín (German name Theresienstadt), located in occupied part of the Czech Republic, called Sudeten. During World War II it served as a Nazi concentration camp staffed by German Nazi guards.

Tens of thousands of people died there, some killed outright and others dying from malnutrition and disease. More than 150,000 other persons (including tens of thousands of children) were held there for months or years, before being sent by rail transports to their deaths at the Treblinka and Auschwitz extermination camps in occupied Poland, as well as to smaller camps elsewhere.Approximately 144,000 Jews were sent to Theresienstadt. Most inmates were Czech Jews, but 40,000 were from Germany, 15,000 from Austria, 5,000 from the Netherlands, and 300 from Luxembourg. In addition to the group of approximately 500 Jews from Denmark, Slovak and Hungarian Jews were deported to the ghetto. 1,600 Jewish children from Białystok, Poland, were deported from Theresienstadt to Auschwitz; none survived. About a quarter of the inmates (33,000) died in Theresienstadt, mostly because of the deadly conditions, which included hunger, stress, and disease. The typhus epidemic at the very end of war took an especially heavy toll.

Theresienstadt  in The War That Came Early
The conquest of Czechoslovakia was accompanied by random attacks on Jews which the conquerors encountered. Later on, all Czech Jews were forced to leave their homes and move into the old fortress city of Theresienstadt.

Literary note
There are no further references to  Theresienstadt in later parts of the series. Assuming that conditions in this  Theresienstadt were comparable to those in OTL, - but given that in this timeline there was no  Auschwitz and other  camaps for the outright extermination of Jews - it can assumed that about a quarter of the  Czech Jews would have perished becuase of the conditions of imprisonment, and three quarters would have survived until the fall of Hitler. It can be assumed that the Comittee for National Salvation - which abolished the anti-Jewish laws in Germany itself and which mangaed to retain German rule in the former  Czechoslovakia - would have allowed these surviving  Czech Jews  to leave  Theresienstadt and return to their homes.