I just can't wrap my head around holocaust deniers and even just straight up world war 2 deniers. All I've heard is a statement that HCN is actually not lethal and couldn't have been used to.. well. But in our modern day and age how can someone try to oppress such a vital and hugely influential part of history?
There only is proof for the Holocaust happening, nothing in the opposite direction.
Holocaust deniers or "revisionists" as they like to label themselves (for a very good discussion on the term and its usage see a recent post here where /u/baronzaterdag explains the term) use misconceptions or cases, which are hard to explain without delving deep into the research that has been done and twist them as to appear credible.
They cherrypick and present things in a way that to someone who is either seeking to validate their opinion or does know very little about the subject at hand, they will appear convincing.
Every misconception Holocaust deniers cherrypick can be explained through historical research and no denier has ever presented any sort of evidence that is not wrapped into some kind of conspiratorial narrative that relies on non-proofable assumptions.
To give you an insight into how deniers work:
So far no written Hitler order for the Holocaust has turned up and most historians of the field believe there never was one. Deniers will take this tidbit and from there will argue that because there is no written order that either the Holocaust never happened or - as David Irving does - that Hitler didn't know about it. They will ignore the wealth of evidence that exists for the Holocaust such as the Wannsee Protocols or the Korherr Report among others and latch onto the fact of the missing order to distort the whole narrative. The reason why there is no written order can be explained however. Because of the problems that arose from the written order of the T4 program, the decision was made among the Nazi leadership to issue the order for the Holocaust orally and while there are several theories when exactly this happened, this is massively supported by historical evidence.
The Nazis themselves used language intended to obscure what they did in their own documents. In the Wannsee Protocols, the Korherr Reports and so on, they often talk about "Sonderbehandlung" (special treatment) or "resettlement to the East" when they really mean killing people. Holocaust deniers will point to this and go "well, they were only resettling people" and ignore the wealth of historical evidence surrounding these documents such as Himmler writing a memo not to use the term "Sonderbehandlung" anymore because by 1943 everyone knew what was meant or the Einsatzgruppen reports, which clearly state that millions of Jews were shot and don't even bother with any of that language. Deniers will twist one piece of historical evidence, which can be easily explained, to fit their narrative.
One thing deniers will argue often is that when looking at the gas chambers in Auschwitz that either they weren't used at all or that they didn't have the capacity to kill 6 million people. The first argument is easily debunked simply by looking at the historical evidence. The second one relies on the common misconception that all victims of the Holocaust were gassed in Auschwitz. They were not. Approx. half of all Jewish victims of the Holocaust were gassed but most of them in the Reinhard camps, Sobibor, Treblinka, and Belcez, as well as the Kulmhof camp while the rest was killed in Auschwitz and Majdanek. The other half were shot by Einsatzgruppen or killed by some other method or died of starvation, disease etc. etc. Deniers however will ignore that and take one misconception and build their narrative upon it. Similarly, they'll take one little piece of evidence such as the unclear usage of the Dachau Gas Chamber and argue that because in one case usage is unclear, it didn't happen in all cases.
These are just some examples but they should give you an insight into how these people work. What drives them to deny the Holocaust is not some piece of historical evidence or some doubts about historiogrpahy and research but a political agenda. Their political agenda is the moral and historical rehabilitation of the Nazi regime because either they themselves are Nazis or they are anti-Semites or they are just conspiracy loons. Their concern is not rooted in historical science but a reprehensible political agenda and no amount of actual historical evidence will stop them from spreading their vile and reprehensible lies in an effort to advance their hateful and inhumane political agenda.
Sources:
Evans, Richard J. Lying About Hitler: History, Holocaust, and the David Irving Trial. New York: Basic Books, 2001.
Gottfried, Ted. Deniers of the Holocaust: Who They Are, What They Do, Why They Do It. Brookfield, CT: Twenty-First Century Books, 2001.
Lipstadt, Deborah. Denying the Holocaust: The Growing Assault on Truth and Memory. New York: Free Press, 1993.
Shermer, Michael, and Alex Grobman. Denying History: Who Says the Holocaust Never Happened and Why Do They Say It? Berkeley: University of California Press, 2000.
Zimmerman, John C. Holocaust Denial: Demographics, Testimonies, and Ideologies. Lanham, MD: University Press of America, 2000.
We also have put some effort in a more general response to these questions:
As this question pertains to basic, underlying facts of the Holocaust, I hope you can appreciate that it can be a fraught subject to deal with. While we want people to get the answers they are looking for, we also remain very conscious that threads of this nature can attract the very wrong kind of response. As such, this message is not intended to provide you with all of the answers, but simply to address some of the basic facts, as well as Holocaust Denial, and provide a short list of introductory reading.
##What Was the Holocaust?
The Holocaust refers the genocidal deaths of 5-6 million European Jews carried out systematically by Nazi Germany as part of targeted policies of persecution and extermination during World War II. Some historians will also include the deaths of the Roma, Communists, Mentally Disabled, and other groups targeted by Nazi policies, which brings the total number of deaths to ~11 million. Debates about whether or not the Holocaust includes these deaths or not is a matter of definitions, but in no way a reflection on dispute that they occurred.
##But This Guy Says Otherwise!
Unfortunately, there is a small, but at times vocal, minority of persons who fall into the category of Holocaust Denial, attempting to minimize the deaths by orders of magnitude, impugn well proven facts, or even claim that the Holocaust is entirely a fabrication and never happened. Although they often self-style themselves as "Revisionists", they are not correctly described by the title. While revisionism is not inherently a dirty word, actual revision, to quote Michael Shermer, "entails refinement of detailed knowledge about events, rarely complete denial of the events themselves, and certainly not denial of the cumulation of events known as the Holocaust."
It is absolutely true that were you to read a book written in 1950 or so, you would find information which any decent scholar today might reject, and that is the result of good revisionism. But these changes, which even can be quite large, such as the reassessment of deaths at Auschwitz from ~4 million to ~1 million, are done within the bounds of respected, academic study, and reflect decades of work that builds upon the work of previous scholars, and certainly does not willfully disregard documented evidence and recollections. There are still plenty of questions within Holocaust Studies that are debated by scholars, and there may still be more out there for us to discover, and revise, but when it comes to the basic facts, there is simply no valid argument against them.
##So What Are the Basics?
Beginning with their rise to power in the 1930s, the Nazi Party, headed by Adolf Hitler, implemented a series of anti-Jewish policies within Germany, marginalizing Jews within society more and more, stripping them of their wealth, livelihoods, and their dignity. With the invasion of Poland in 1939, the number of Jews under Nazi control reached into the millions, and this number would again increase with the invasion of the Soviet Union in 1941. Shortly after the invasion of Poland, the Germans started to confine the Jewish population into squalid ghettos. After several plans on how to rid Europe of the Jews that all proved unfeasible, by the time of the invasion of the Soviet Union in 1941, ideological (Antisemitism) and pragmatic (Resources) considerations lead to mass-killings becoming the only viable option in the minds of the Nazi leadership. First only practiced in the USSR, it was influential groups such as the SS and the administration of the General Government that pushed to expand the killing operations to all of Europe and sometime at the end of 1941 met with Hitler’s approval.
The early killings were carried out foremost by the Einsatzgruppen, paramilitary groups organized under the aegis of the SS and tasked with carrying out the mass killings of Jews, Communists, and other 'undesirable elements' in the wake of the German military's advance. In what is often termed the 'Holocaust by Bullet', the Einsatzgruppen, with the assistance of the Wehrmacht, the SD, the Security Police, as well as local collaborators, would kill roughly two million persons, over half of them Jews. Most killings were carried out with mass shootings, but other methods such as gas vans - intended to spare the killers the trauma of shooting so many persons day after day - were utilized too.
By early 1942, the "Final Solution" to the so-called "Jewish Question" was essentially finalized at the Wannsee Conference under the direction of Reinhard Heydrich, where the plan to eliminate the Jewish population of Europe using a series of extermination camps set up in occupied Poland was presented and met with approval.
Construction of extermination camps had already begun the previous fall, and mass extermination, mostly as part of 'Operation Reinhard', had began operation by spring of 1942. Roughly 2 million persons, nearly all Jewish men, women, and children, were immediately gassed upon arrival at Bełżec, Sobibór, and Treblinka over the next two years, when these "Reinhard" camps were closed and razed. More victims would meet their fate in additional extermination camps such as Chełmno, but most infamously at Auschwitz-Birkenau, where slightly over 1 million persons, mostly Jews, died. Under the plan set forth at Wannsee, exterminations were hardly limited to the Jews of Poland, but rather Jews from all over Europe were rounded up and sent east by rail like cattle to the slaughter. Although the victims of the Reinhard Camps were originally buried, they would later be exhumed and cremated, and cremation of the victims was normal procedure at later camps such as Auschwitz.
##The Camps
There were two main types of camps run by Nazi Germany, which is sometimes a source of confusion. Concentration Camps were well known means of extrajudicial control implemented by the Nazis shortly after taking power, beginning with the construction of Dachau in 1933. Political opponents of all type, not just Jews, could find themselves imprisoned in these camps during the pre-war years, and while conditions were often brutal and squalid, and numerous deaths did occur from mistreatment, they were not usually a death sentence and the population fluctuated greatly. Although Concentration Camps were later made part of the 'Final Solution', their purpose was not as immediate extermination centers. Some were 'way stations', and others were work camps, where Germany intended to eke out every last bit of productivity from them through what was known as "extermination through labor". Jews and other undesirable elements, if deemed healthy enough to work, could find themselves spared for a time and "allowed" to toil away like slaves until their usefulness was at an end.
Although some Concentration Camps, such as Mauthausen, did include small gas chambers, mass gassing was not the primary purpose of the camp. Many camps, becoming extremely overcrowded, nevertheless resulted in the deaths of tens of thousands of inhabitants due to the outbreak of diseases such as typhus, or starvation, all of which the camp administrations did little to prevent. Bergen-Belsen, which was not a work camp but rather served as something of a way station for prisoners of the camp systems being moved about, is perhaps one of the most infamous of camps on this count, saw some 50,000 deaths caused by the conditions. Often located in the Reich, camps liberated by the Western forces were exclusively Concentration Camps, and many survivor testimonies come from these camps.
The Concentration Camps are contrasted with the Extermination Camps, which were purpose built for mass killing, with large gas chambers and later on, crematoria, but little or no facilities for inmates. Often they were disguised with false facades to lull the new arrivals into a false sense of security, even though rumors were of course rife for the fate that awaited the deportees. Almost all arrivals were killed upon arrival at these camps, and in many cases the number of survivors numbered in the single digits, such as at Bełżec, where only seven Jews, forced to assist in operation of the camp, were alive after the war.
Several camps, however, were 'Hybrids' of both types, the most famous being Auschwitz, which was vast a complex of subcamps. The infamous 'selection' of prisoners, conducted by SS doctors upon arrival, meant life or death, with those deemed unsuited for labor immediately gassed and the more healthy and robust given at least temporary reprieve. The death count at Auschwitz numbered around 1 million, but it is also the source of many survivor testimonies.
##How Do We Know?
Running through the evidence piece by piece would take more space than we have here, but suffice to say, there is a lot of evidence, and not just the (mountains of) survivor testimony. We have testimonies and writings from many who participated, as well German documentation of the programs. This site catalogs some of the evidence we have for mass extermination as it relates to Auschwitz. I'll close this out with a short list of excellent works that should help to introduce you to various aspects of Holocaust study.
##Further Reading