So, once upon a time in Germany, around 1931, Reinhard Heydrich (the father of the genocide machine to later feature in the Holocaust) was tasked with starting the Sicherheitsdienst, sister agency to the Gestapo. See, Germany was going through a hard time, and while the communists wanted to try out that business Lenin was doing, the companies and interest behind the NSDAP wanted the people to fall behind an autocratic regime that would take care of them and make sure everyone stayed in their lane.

And Heydrich (who was still new at it but read a lot of cloak-and-dagger spy stories) worked out that the enemy within rhetoric served to keep the German people lawful. At first he started rounding up the wierdos (gays, sexual perverts, street beggars). Since no one liked the Jews (really, the whole world was on an antisemitic bender), he added them to the list. Then the communists. Then the trade unionists. And then everyone employed in a job that he thought wasn't serving the state very well. He really got going.

The first concentration camp was a warehouse in Berlin, but it filled up super fast, and in time he needed bigger spaces, and so new camps weren't just ad-hoc corrals but ghettos and more permanent prison camps for the growing contained population, in what was becoming The Jewish Problem: There were too many people to keep in ghettos and camps and (remember that global antisemitism) no-one else was willing to take Jewish refugees. Not even the United States. If only there was a way to make them disappear...