Holocaust efficacy questions are disgusting in my opinion.

From a previous answer:

Part 1

Several things need to be taken into account for an answer here.

First, not all "undesirables" were to be systematically killed. Political enemies, so-called "asocials", criminals, homosexuals, Jehovah's Witnesses were first and foremost to be imprisoned and secondly to be exploited economically, especially after the two waves of Concentration Camp economization in 1938 and 1942.

Many of them dies and suffered horribly but for the Nazis, these people were not on the list to be systematically murdered wholesale. Amidst the general lack of work force in the Nazi war economy, the labor of Concentration Camp prisoners became a rather valuable resource, in the sense that the SS started their own companies (mostly in 1938) and later on started to rent or lease Concentration Camp labor to big companies in the Third Reich. In the Auschwitz camp complex, Auschwitz III Monowitz was a camp that basically existed only to provide the IG Farben -- the biggest chemical company in the world at that point -- with cheap prison labor. Of course, the working and living conditions were horrifying and thousands upon thousands died of starvation, maltreatment, disease, and of being executed. But the initial plan was not to have them killed wholesale.

Four particular groups were singled out by the Nazis to be killed wholesale: Physically and mentally disabled within the borders of the German Reich, who were in corresponding institutions, Jews, so-called gypsies (Roma, Sinti and others who were caught under the umbrella term), and Soviet POWs in 1941.

With Soviet POWs, the initial plan was to let them starve in order to save food to feed the German Wehrmacht and the German populous. In 1941 alone 1.5 million of a total of 3 million Soviet POWs stared to death and all in all about 3.3 million Soviets in German captivity died throughout the war out of a total of 6 million. Starvation was chose here because it was the "easiest" route to take and it didn't require anything more than putting them in camps and watch them die. This policy however changed when it became apparent that the Germans weren't going to win the war in the Soviet Union in 1941. Due to the labor shortage the war caused for the German economy, it was decided that the should work for the Germans rather than starve wholesale. And while, in later years, there still occurred mass deaths of Soviet POWs in Germany, this was more the result of a policy of intentional neglect of their basic human needs, i.e. indirect mass murder, rather than a campaign of seeking them out to kill them.

For the mentally and physically handicapped, the rationalization of the Nazis to kill these groups was ideological -- to get them out of the gene pool -- as well as economical -- to not have to feed and care for them anymore. But because this group was mainly German citizens with German relatives and the Nazis were highly conscious of popular support during the war, the initial program set up to kill these "unproductive eaters", the so-called T4 program, was done in secrecy. Shooting them was a highly public act, the German population would have noticed. This program is where the method of gassing as a way to mass kill people originated.

Two things are important here: First of all, when Hitler, Philipp Bouhler of the Reich chancellery, and Karl Brandt, Hitler's doctor, decided to bring the T4 program underway in 1939, they decided that only those within appropriate institutions (hospitals, homes, and so on) were to be killed because arresting the handicapped in their homes dragging them away from their families directly ran contrary to their need for secrecy. Secondly, because of said need for secrecy, shooting was out of the question. Therefore, they asked the Criminal Technical Institute of the Security Police (Kriminaltechnisches Institut der Sicherheitspolizei, KTI) what the fastest, most-effective, and time-saving method of mass killing was, they could come up with. The KTI ran several experiments, first on animals, but then in October 1939 they also experimented for the first time on mentally ill people in Posen, where about 400 people were gassed with Carbon-monoxide from gas bottles. Witnessed by August Becker, later to be the inspector for the Gas Vans in the Soviet Union, the KTI was apparently satisfied with that method, especially with how it was possible to kill a lot of people in a short amount time with a resources that was easily available because every engine could produce it. Thus they recommended this method to the planners of T4.

During 1940 the T4 program also conducted their own trials of which method of killing was the most effective. While gas chambers and crematoria were established in six main T4 facilities -- Brandenburg, Grafeneck, Hadamar, Bernburg, Hartheim, Sonnenstein --, the worries about keeping the program secret were well founded on the side of the Nazis because when the Catholic Church started publicly preaching about the injustice of the program, the Nazi government saw itself forced to publicly stop the program. While the killing of the mentally and physically handicapped became a decentralized program with individual doctors killing their own patients instead of sending them to one of the central facilities, the gas chambers in the T4 facilities were still used for program 14f13. 14f13 was the deliberate killing of Concentration Camp inmates who had become to weak or sick to work. The name comes from the Concentration Camps' forms on causes of death. 14f13 stood for death by diseases and during some phases it can be read by historians today as proof that the people were gassed.

For Jews and the so-called gypsies, the development of policy was different. The first mass executions of Jews took place in Poland in 1939 when the Nazi Einsatzgruppen killed Polish intellectuals and Jewish political enemies in the thousands as part of their campaign to deprive the Polish people from any sort of intellectual leadership that could lead the Polish resistance or a resurgence of the Polish nation.

But the first systematic mass murder of Jews took place in the Soviet Union after the invasion of June 1941. Before the invasion, the German High Command in collaboration with the SS had decided that the Einsatzgruppen had the purpose of rooting out all communists in the Soviet Union. This was specified in the so-called Commissar's Order. When German troops crossed the Soviet border, the EInsatzgruppen set to work immediately. Because in the warped logic of the Nazis, all Jews were agents of communism resp. the puppet-masters of communism in order to subdue the Soviet Union, all of them had to be killed. From Summer to December 1941, the Einsatzgruppen roamed the Soviet Union, seeking out all Jews they could find and systematically shooting them. Within the span of a couple of months, the Einsatzgruppen had killed 1.5 million Jews by shooting, which is about a quarter of all Jewish victims of the Holocaust. They also killed thousands upon thousands of so-called gypsies during these actions.

It was also the experiences during these Einsatzgruppen actions that lead to the method of deportation and gassing. When it emerged during autumn 1941 that all Jews of Europe should be killed in a systematic fashion, Himmler visited one of the mass executions. Apparently he was horrified by what he saw, especially by the impact these mass executions, sometimes taking days, had on the men of the Einsatzgruppen. He feared this would drive them into demoralization and alcoholism. So he order a method that was more humane for the executioners to be found. Once again, the leadership of the Reich Sicherheit Hauptamt turned to KTI to develop new methods of execution. After some experiments, including blowing people up with explosives, the KTI once again recommended Carbon-monoxide gassing as the "best" method to go because for the most part, it was possible to kill a lot of people relatively fast and it spared executioners having to witness the consequences of their actions for the most part.

The second factor that plays into this was that the T4 program had to be stopped around that time. So the Nazi regime suddenly found itself with about 400 people experienced with gas killings who needed a new task. From November 1941 forward a lot of the former T4 officials went on to other places with gas vans. Every Einsatzgruppe received one and there was also one in Riga, while the Sonderkommando Lange went on to construct the death camp at Chelmno/Kulmhof designed to kill the inhabitants of the Lodz Ghetto. Another gas van was also sent to Serbia, where the Wehrmacht had already killed the male Jews of the country in the course of anti-Partisan warfare and the women, children, and elderly were also slated to be murdered.

With the decision to kill all the Jews of Europe taken at some point in December 1941, the planners of the Nazi genocide found themselves with the task to kill millions of Jews in a fast, effective, and cost-effective fashion. They again decided because of their "good" experiences with the gas van on gas as the preferable method but because of the massive numbers of victims decided on stationary gas chambers. In the camps of the Aktion Reinhard, the killing of the Polish Jews from summer 1942 to spring 1943, they opted for gas chambers attached to Russian tank engines producing Carbon-monoxide. In about 9 months, they killed over 1.5 million people this way, all run by the former T4 program experts. These about 400 people managed virtually all three Reinhard Camps, Sobibor, Treblinka, and Belzec, where this took place.