The rest of the world found out what was really happening through detailed records and even liberating some of the camps.
My question is: in the several million men that served in the land forces, the Kriegsmarine, and the Luftwaffe, how many knew about what was happening? Was it more likely that they got called to duty and drafted to fight a big war so that's what they did (even though they had to recite an oath to Hitler)?
If the general armed forces DID know, then was it just the officers or was it possible that a soldier on the frontlines knew too? Is it even possible to know without asking men who served in the German army at that time?
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Of course the armed forces knew. And not just the general staff or officers, virtually every one of the Wehrmacht's 18 million soldiers had either knowledge of the genocide against the Jews or directly participated in the Holocaust.
First off, it is important to take a look at the Wehrmacht as an institution of the Nazi state. As is well known, via the dictum of Clausewitz, war is the continuation of politics by different means. For the Nazi regime and its army, war was definitely the continuation of a racist and genocidal politics by military means. And the Wehrmacht as an institution as well as its individual members from the general staff involved in planning and strategizing such a war down to its individual soldiers played a pivotal role in making the vision of a racist German-lead New Order with Europe's Jews dead a – partial – reality.
Every institution of the Nazi state, and especially the Wehrmacht, were complicit in Nazi crimes. To exemplify (the list I am providing is far from extensive):
The probably most famous examples of Wehrmacht crimes are probably the Commissars Order and the Kriegsgerichtsbarkeitserlass. When preparing for the invasion of the Soviet Union, the Wehrmacht leadership in conjecture with the Nazi leadership issued orders that the war in the Soviet Union was not to be treated as a "normal" war but a war of "Weltanschauung", meaning they were not just fighting another country but rather Jewish-Bolshevism itself. To that end, the OKW gave the order that political commissars within the Red Army were not to be treated as POWs but were to be shot immediately after capture. Political Comissar included however not only people who held this position but also any member of the Communist Party of the Soviet Union as well as all Jews. In conjecture with this order, the Kriegsgerichtsbarkeitserlass decreed that no member of the Wehrmacht could be persecuted for any and all war crimes they committed while in the Soviet Union. So rape, pillaging, murder and burning down villages were all fair game for all members of the Wehrmacht. The Commissar Order alone lead to something between 60.000 and 140.000 victims.
Additionally, the Wehrmacht as an institution was responsible for Soviet POWs in general. In that function it was the Wehrmacht which basically let them starve to death in violation of all international treaties and conventions. Basically, the Wehrmacht built POW camps for Soviets by just putting up a fence and putting the POWs in there, letting them starve as a policy before the leadership of Nazi Germany needed them for forced labor in 1942. But even with that the death toll is staggering. Christian Streit estimates that about 3.3 million Soviet POWs or 57% of all Soviet POWs captured by the Wehrmacht died while in captivity.
The Wehrmacht was an important part of the occupation of conquered territory and as a security force in that occupied territory. As such, it committed murder and war crimes. Taking Serbia as a territory that was directly administered by the Wehrmacht, Wehrmacht units shot 20.000 civilians alone in the time frame from September to December 1941 as part of a campaign of retaliation for Partisan attacks. The Wehrmacht commander of said territory, Franz Böhme, instituted a policy of 100 civilians shot for every dead German soldier and 50 for every injured German soldier. The vast majority of victims were not related to the attacks or the Partisans but rather male Jews or Roma and Sinti thus making Serbia the first territory outside of the Soviet Union in which Jews were systematically killed by the German occupation.
Crimes such as these are numerous and extend even into the Western territories of Europe. For example, the Wehrmacht massacre of the Italian village of Marzabotto in October 1944.
Also, as an occupational force, the Wehrmacht was responsible for administering Nazi racial policy in its territories as can be read in detail in Dieter Pohl's book on the Wehrmacht in the Soviet Union.
As mentioned above, the Wehrmacht as an institution was involved in the Holocaust in Serbia, where it was Wehrmacht untis who killed the male Jewish population or when it came to Soviet Jewish POWs. But the Wehrmacht also collaborated closely with the Einsatzgruppen in the Soviet Union basically either transferring Jews into the hands of these mobile killing units or even lending a hand when it came to shooting Jews. Additionally, Wehrmacht units in Poland and the Soviet Union also were involved in killing the mentally handicapped and disabled.
Furthermore, the Wehrmacht established Ghettos and provided transport for Jews to be deported to Auschwitz, e.g. in France and aided in registering and confining Jews to certain quarters in countries such as France and Belgium.
The Wehrmacht in fact encouraged its troops to use massive violence against a civilian population as a legitimate means to an end. Dating back to the Franco-Prussian war of 1871, the German army was almost notorious for its fear and hard line against Franc-Tireurs and irregular fighters. The German military doctrine was to employ hard reprisals against the civilian population harboring irregular fighters and security threats. This was allowed by the Hague convention and also used in accordance of it during for example WWI but it was a line of thinking that was quickly adapted to Nazi ideology, meaning that it was applied to Jews ("Where there is the Jew, there is the Partisan, and where there is the Partisan, there is the Jew" as the line from Erich von dem Bach-Zelewski ran) and other civilians perceived as a security threat.
I have mentioned the Serbian example above but this is a general phenomenon in Wehrmacht occupation troops. Rather than punishing violence against civilians, numerous examples prove that the Wehrmacht encouraged violence. Klaus Michael Mallmann for example mentions in one of his articles that a Wehrmacht security unit in Poland was shown the movie "Jud Süß" and became so enraged by it and the copious amounts of alcohol they had consumed that they went out and hunted and shot all the Jews of the town they were stationed in.
This is important because when it comes to your question, it is imperative to know that the Wehrmacht as an institution itself, regardless of what some individual members did or thought, was complicit in Nazi crimes and committed crimes itself.
As for the knowledge of individual soldiers, new research by both Harald Welzer / Sönke Neitzel and Felix Römer give us insight in that. They worked extensively with Allied protocols of conversations between German POWs recorded in Allied camps when they didn't think anybody was listening. Their research uncovered that knowledge of war crimes was ubiquitous among members of the Wehrmacht. Every soldier knew of atrocities that had been committed against Jews and other civilians because they had either been present, had participated or had been told about them by their comrades. During their time as POWs, they quite freely discussed these crimes. To exemplify this, Römer cites among others the following exchange between the Viennese Artillerie-Gefreitem Franz Ctorecka and the Panzer-Gefreiten Willi Eckenbach in August 1944 in Fort Hunt (translation my own):
C: And then Lublin. There is a crematoria, a death camp. Sepp Dietrich is involved there. He was somehow caught up in this in Lublin.
E: Near Berlin, they burned the corpses in one of these thingies ["einem Dings], the people were forced into this hall. This hall was wired with high-voltage power-lines and in the moment they switched on these lines, the people in the hall turned to ashes. But while still alive! The guy who was in charge of the burning told 'em: "Don't be afraid, I will fire you up!" He always made such quips. And then they found out that the guy who was in charge of burning the people also stole their gold teeth. Also other stuff like rings, jewellery etc.
[Römer, p. 435f.]
The conduct of the Final Solution was not exclusively reserved for the SS. It was a clear policy of the Nazi state and the Wehrmacht were explicitly involved in war crimes including the extermination of Jews, Slavs and other “undesirables”.
Firstly the SS were not solely the secret police of the Nazi regime, that function was specifically performed by the Geheime Staatspolizei, known universally as the Gestapo. The Gestapo reported to senior SS leaders. The SS in general was the paramilitary wing of the Nazi party which, through the establishment of the Waffen SS, became a de facto military force in addition to a wide range of other responsibilities. Thanks to the chaotic way in which the Nazi party, and ultimately the Nazi state, was organised agencies often found themselves performing similar or overlapping functions. The Gestapo would often overlap with the SD (the Nazi Secret Service, also part of the SS), the Waffen SS clearly overlapped with the Wehmracht and the Wehrmacht overlapped with the Gestapo.
As the standing military force of the Nazi state, the Wehrmacht was intimately involved in Nazi policy. As an occupying force in Western Europe it was responsible for implementing security policy alongside the SS/Gestapo. This meant that it was responsible for rounding up suspected partisan, enforcing reprisals against civilians and supporting the deportation of Jews. In Southern Europe the Wehrmacht was responsible for anti-partisan operations which included reprisals against civilians including a large number of massacres.
In Eastern Europe the Wehrmacht was instructed that it was to engage in a war of annihilation (Vernichtungskrieg) and that the invasion of Russia was intended to destroy “Judeo-Bolshevism”. As well as distinct military tasks, the Wehrmacht was given powers to conduct summary justice for civilians. This meant that the Wehrmacht was free to arrest and punish civilians as it saw fit – often leading to summary executions, torture and rape. It was also responsible for filtering out undesirables amongst POWs and the civilian population and was free to execute them. Given that the Nazi plan for Eastern Europe (General Plan Ost) expected to wipe out 95% of Slavs and enslave the rest, this gave the Wehrmacht free rein to brutalise the population. German soldiers were specifically told that their opponents were subhuman.
The important point in all of this is the use of the word “explicit”. We know that the Wehrmacht were not unwitting accomplices to a war of genocide because we have the orders they were given. It wasn’t a case of “a few bad apples” because the Wehrmacht’s tasks went far beyond engaging with enemy combatants in accordance with the laws of armed conflict. We also know that this wasn’t a case of “just following orders” because we have letters sent from the front by German soldiers not only boasting of their actions, but using the language of Nazi Genocide: “untermensch”, “asian mongrels”, “judeo-bolshevik scum” and so on.
In recent history the popular understanding of the Wehrmacht in World War 2 has been clouded by deliberate attempts to cover up, or gloss over, crimes in order to rehabilitate and re-establish the German armed forces. This was a deliberate West German policy supported the Western Allies and is referred to as the “myth of the clean Wehrmacht”, I’ve written more about it here:
https://www.reddit.com/r/AskHistorians/comments/6he40y/wehrmacht_vs_ss/
/u/commiespaceinvader is a font of knowledge on Nazi Germany and talks about the distinction between Nazis and “ordinary Germans” here:
And more detail about Wehrmacht crimes here:
With another post by /u/Astrogator here:
There are also a series of excellent posts included in the FAQ: