Reinhard Heydrich was one of the main heads behind the Holocaust. After the mass extermination of millions of people was planned at the Wannsee conference, Heydrich was assassinated. The operation was named after Heydrich's first name, since he had been central to organizing the mass murder and general oppression in Europe.
Three death camps with gas chambers were constructed during Aktion Reinhard: Treblinka, Sobibór and Belzec. At least 1,7 million people were exterminated in these camps, either by suffocating them with diesel motor exhaust, or poisoning them with Zyklon B gas, as happened as well in the better known death camp of Auschwitz-Birkenau.
Aktion Reinhard was devised as a more "humane" method of mass murder, but only to be more "humane" to the murderers conducting the act of killing. Mass executions with rifles or MGs had a rather adverse effect even on the mental stability and conscience of SS execution units. Aktion Reinhard utilized gas for killing people to abstract the act of killing. It was the start of an industrialized mass murder formerly not known to mankind.