Dachau

As soon as the Nazis came to power in Germany, they immediately began persecuting their political opponents. The first concentration camps like Dachau were established for this purpose.
The first transports of prisoners arrived at a disused gunpowder and munitions factory on the outskirts of the town of Dachau, north of Munich, on March 22, 1933, two months after Adolf Hitler was appointed «Reichskanzler» (Chancellor of Germany).
The first prisoners to arrive at Dachau were political opponents of the Nazis (e.g., communists), who were arrested en masse.

Although the Dachau camp also had a gas chamber labeled «Shower», no one was gassed there. People starved, died from deceases or in sadistic medical experiments, were shot or hanged.

Dachau was the first concentration camp I ever been to. I visited this site on Oct 15, 2014.