The gist of the answer here is that within the völkisch nationalist roots of Nazism as an ideology, thed European East, especially Poland and Russia / the USSR were viewed as Germany's "assigned" colonial space akin to the British and French domain in Africa. Therefore these lands could and should be settled and its inhabitants subjugated – the subject matter of the GPO – while Greece or Belgium were not assigned the same role in this world view because of real and imagined historical differences.
In general , historiography has moved away from talking about Slavs and Nazis in general, in favor of a more differentiated approach of talking about Poles and Soviets. One of the reason is the difficulty of definition that /u/marisacoulter details here and the second one is detailed by John Connely in his article Nazis and Slavs: From Racial Theory to Racist Practice. Central European History, Vol. 32, No. 1 (1999), pp. 1–33 which discusses that aside from general declarations of some party leaders, the category of "Slav" as an overarching idea for all speakers of Slavic languages did not play a role in the implementation of concrete policy in the occupied territories. Czechs, Poles, Russians, Serbs and so forth were viewed so distinctly different by the Nazis and treated distinctly different that the category of "Slav" is not a useful one in illuminating that history. It can not explain why Czechs and Slovaks were treated differently or why Croatia was turned into a satellite state and Serbia was occupied or why in Poland "carriers of Polish national sentiment" were murdered en masse while in Serbia they weren't to the same extent and degree. In the following however I'll do my best to talk comprehensively.
So, according to Nazi racial ideology, Slavic peoples were "inferior". On the racial scale the Nazis set up in their ideology Slavs occupied one of the lowest positions, just above Jews and so-called gypsies. Similar to the anti-Semitism of the Nazis having certain historical roots as described here, the anti-Slav racism of the Nazis also goes back to earlier stereotypes and antipathies.
Similar to anti-Semitism in its form practiced by the Nazis being a result of seeing race as a driving factor in history while referencing earlier / already common stereotypes and prejudices, the Nazis' anti-Slavism also originated from the attempt to explain how the world works and what the state of the world is through the lens of racial theory. Already in the 19th century (and probably earlier but that falls a bit outside my area of expertise), views of the Russian people and by extension in the 19th century, the Slavic people had a certain negative bend in Germany. Russia and its people were seen as backward peasants that missed the entry into modernity because of their "archaic" mindset, something amplified by the fact that the Russian Tsars in the second half of the 19th century took a viewpoint that was not entirely dissimilar and which in fact spawned certain reform attempts from up top (trying to abolish the last remnants of the feudal system in Russia e.g.).
Also, a strong presence within the German mindset about the Slavic people were the Poles. Large swaths of Poland were at that time part of German/Prussian territory and within the social set-up of it, the Poles occupied a socially inferior position with German / Prussian junkers owning land and Poles toiling it for them. This further shaped the perception of the Slavic people being a people / "race" that was predestined to serve its German masters as subservient.
The other important ideological strand, we can not neglect when talking about Nazi ideological perception of Slavic people is the Austrian situation since Hitler as well as other important Nazis were Austrian and strongly influenced by the political and ideological situation in the Austro-Hungarian monarchy at the time. The Austro-Hungarian monarchy throughout the second half of the 19th century and leading into the 20th century experienced strong conflicts based on a newly emergent nationalistic sentiment in its Slavic (Czech), South Slav (Slovenian, Croatian, and from 1878 on Bosnian), Hungarian, and German population with conflicts emerging surrounding the use of language, the question of political representation, and the German-speaking dominance of the state administration.
What really once again racketed up the level of the negative perception of Slavic people among the German-speakers of Austria and Germany was WWI. In the Austrian case, the pretty obvious negative stereotypes against the Serbs as well as against its own Slavic population that was suspected of harboring sympathies for the Serbian cause. The letter even lead to the Austrian government deporting hundreds of thousands of its own Slovenian and Croatian citizens away from their homes near the Southern border to cities like Linz and others and interning them there in camps. For the Germans, there was of course the old stereotypes of the backward and inferior Russians, which were heavily solidified by the experiences of German soldiers in Russia. Seeing the abject poverty many of the Russian subjects lived in while marching through their country gave a lot of people the impression that they were essentially a people living in filth and neglect.
With the Bolshevik revolution occurring in Russia in 1917, anti-Slavic sentiments among many of the racist early »völkisch« (racialist, viewing the world and history through the lens of a supposed race conflict) thinkers combined anti-Salvic and anti-Semitic sentiment with anti-Bolshevism. The outcome for them was the ideological formation that communism was the tool of »international Jewry« and the Slavs its expandable vanguard. This was highly influential for the Nazis. In essence, they saw the Jews as the puppet masters of international Bolshevism that sought to impose its rule through the »asiatic barabrity« and »eastern despotism« of the Slavic people. This Nazi propaganda poster gives you an impression of that. The depicted Commissar displays features attributed to Jews as well as Russians at the time and the depiction of the massacre below serves to portray their barbarity and cruelty.
Maria Toderova, a highly regarded scholar of South Eastern Europe, speaks in connection to the Western European perception of the Balkans of a phenomenon she dubs »Balkanism«. Balkanism is the othering (i.e. making the not like us in the discourse) of the inhabitants of the Balkans, seeing them as naturally »savage«, »violent«, »uncivilized«, »non-European«, »oriental« in a manner similar Edward Said described in his book on Orientalism for the Middle East. While the concept can not be transferred 1:1 on other Slavic peoples, it still rings true that in the Nazi -- and also wider German perception -- of the Slavs the latter were made out to be inferior, uncivilized, brutal, asiatic, and only fit as a »slave race« (all with a degree of variance, while Russians, Poles, and Serbs were seen as completely inferior, Croats and Bulgarians were due to political necessity displayed in a more positive light, and Slovenes even as »Germanizable«).
This also had very concrete and horrifying political consequences. When the Germans invaded Poland in 1939, the SS Einsatzgruppen immediately started to execute Polish intellectuals, clergy, and politicians by the thousands. The idea behind that was to deprive the Polish people from any future political leaders or intelligentsia so that they could serve as slaves to the German master race. In the Soviet Union, not only did the Nazis plan to let millions of people starve so that they Germans could be fed, in their policies in the occupied territories, they for example forbid any Soviet citizen to get education beyond learning the basics of how to read and write because in the Nazi imagination, they wouldn't need anymore than that. Among the POWs of the Soviet army, those soldiers with »asiatic« features were immediately executed together with Jews and political commissars. Serbia as a country was placed under Wehrmacht administration because the Serbs were perceived as especially violent and treacherous because -- according to the German Wehrmacht commander -- "the Serbs has Ottoman and Slavic blood, the only language he understands is violence". The whole set-up of occupational policies such as placing the Czechs, Poles etc. under German adminsitraiton, not allowing their own bureaucrats within the ranks of the administration in contradiction to for example Belgium was not only because of the political necessity but also because of the racist view of the Slavic people.
In short, the Nazis saw different people subsumed as slavs as inherently inferior, savage, and uncivilized people, who only understood the language of violence. This translated into brutally savage policies of killing unbelievably high number of Slavic people whether it was through direct violence, starvation, or neglect. In the Nazi vision of the New Order, they were to fill the role of colonial slave peoples that had to serve the German master race.
Sources:
Mark Mazower: Hitler's Empire.
Wendy Lower: Nazi Empire-Building and the Holocaust in the Ukraine.
Maria Toderova: Imaging the Balkans.
Dieter Pohl: Die Herrschaft der Wehrmacht in der Sowjetunion.
Robert Gerwarth: The Central European Counter-Revolutionary: Paramilitary Violence in Germany, Austria, and Hungary after the Great War.
The reply by /u/commiespaceinvader already gives a superb account of Nazi attitudes towards the peoples of Eastern Europe and the Balkans. I just want to answer the specific sub-question: what about the Greeks?
The short version is that Nazi views here were a bit more complicated because, like many Nazis in the present day, the Nazis of Hitler's Germany loved Sparta. Loved it, adored it, wanted to be like it. In the words of modern historian Stefan Rebenich, Sparta was part of the Nazi "religious system".^1 In Nazi boarding schools, children were taught to think of themselves as little Spartans, to memorise Spartan poetry, and to emulate Spartan heroism. On his final birthday in his bunker beneath Berlin, Hitler recalled the story of the 300 Spartans at Thermopylai to encourage his followers to fight to the last. With this level of admiration for an ancient Greek community, it's understandable that the modern Greeks had something of a special status among the peoples of the Balkans in terms of Nazi racial categories.
The reason for the Nazis' love of Sparta are easy to see. In the prevailing scholarly view of the time, Sparta was a society of elite farmer-soldiers, selflessly committed to the defence of their homeland, which was worked on their behalf by a ruthlessly suppressed class of enslaved people. It was a society that forbade marriage between citizens and non-citizens and practiced eugenics (through the murder of deformed babies) in order to secure the health and purity of the citizen body. It also subjected its children to an education system that focused on unflinching obedience, unfaltering discipline, and the total subordination of the individual to the state. The ultimate symbol of the value system of this 'pure' society of equals was the story that everyone knew, the last stand at Thermopylai, which had been shared and savoured by German military officers and civilians alike for generations. In short, Sparta could be seen as a community with the same values and social practices that the Nazis applied or would like to apply, and history showed that it had resulted in unmatched heroism and long-lasting Spartan success.
But there was another strand to the Nazi appreciation of Sparta which might seem less obvious to us now. The Ancient Greeks believed that most of their communities did not originate where they were living at the time, but had moved there in one historical migration or another, conquering or displacing the original population of their new homeland. The Dorian Greeks of the Peloponnese, in particular (which included the Spartans), were believed to have migrated south from a homeland in Northern Greece or Macedon in the distant past, subjecting the existing population of the Peloponnese to slavery and ruling over them as an upper class of outsiders. From the influential work of Karl Otfried Müller (1824) onwards, it came to be believed that these outsiders brought many of the Ancient Greeks' famous accomplishments with them, and actually imposed civilization on the formerly barbaric peoples of the Peloponnese. By the early 20th century, some strands of scholarship argued that the homeland of the Dorians wasn't actually in Northern Greece, but much further north. And that they were not swarthy Mediterranean types but blonde, with strong jaws, and pure bloodlines, and...
Basically, they came to believe Spartans were a Nordic people. Nazis believed that the Spartans descended from the same stock as modern Germans, and that it was their Aryan purity and racial superiority, retained through their oppressive laws, that had allowed them to rule over the Greeks as a Herrenvolk over its predestined slaves. Just like modern Germany was by nature destined to rule over the peoples of Eastern Europe.
Feeding on an older intellectual tradition that equated modern Germans with Ancient Greeks - long politically fragmented, but scientifically and philosophically accomplished and possessed of a strong sense of national unity - this distorted and racialised version of Spartan history made the Greeks a fixture in the propagandistic form of history that was taught in schools and universities in the Nazi era. Increasingly, Sparta was painted as a proto-Nazi state, and their stand at Thermopylai as a defence of the Aryan race against its enemies, to be emulated by its modern defenders (you may feel like you saw this particular interpretation in a movie recently). In light of such Nazi propaganda, it seems understandable that the Nazis did not deny the modern Greeks their humanity in the same way that they did other Balkan peoples.
There is a previous answer to a similar question that can be found here by u/commiespaceinvader. While it does not go particularly into why the Greeks were not looked down upon as much as the Slavs were, it explains Hitler position on the Slavs, which like his Jewish position comes from preexisting ideas and racism present in Germany and other parts of Europe.
I had not heard that Baltic peoples were determined to be exterminated. Is there any evidence of this?
Germans had a long history in ruling the area and living with Balts, from carving out Prussia, to the Livonian and Teutonic Knights to the Hanseatic league. Baltic units served with the Wermacht and the SS.
My understanding of this is that while Balts were of course below Germans in the ethnic pecking order, Hitler regarded Balts "as being essentially close to the master race", at least those "of good stock".
http://www.lituanus.org/1965/65_1_03_Slavenas.html
NAZI IDEOLOGY AND POLICY IN THE BALTIC STATES JULIUS P. SLAVENAS
This will probably get removed, but I’d just like to say that this is an amazing thread with plenty of useful in depth answers. I learned a lot, thanks to the users that answered to this question!
