I can do my best to answer it based on what I know, but it would be my first time answering anything on this forum so bear with me.

Why were the Nazis so much more severe in the treatment of the Poles?

Mostly for the same reasons as for their treatment of all the Slav people, most 'famously' the Russians. They considered the Slavs 'untermenschen' where a rough translation would be 'subhuman'. This justified their extreme racial violence towards these groups as the soldiers were told that the Slavs were little more than animals and would become slaves once the German settlers moved into the east after the creation of a German Empire.

There are some specific examples of extreme Nazi aggression towards the poles in both the early and later stages of the war. At the very start, the German soldiers were warned about being shot in the back by Polish resistance and were therefore encouraged to have a 'shoot first, ask questions later' policy. Furthermore, the Nazis officers did not stop their soldiers from exacting terrible revenges on the native Poles after one or two soldiers had been killed by the resistance. In revenge, they might round up 50 poles and kill them to set an example. There are also cases of Soldiers throwing grenades into cellars filled with people as they expected that these civilians would fight back against them.

Does that compare with their treatment of Western European and American prisoners of war

I got very little knowledge on this area but I'll try to answer it anyway. The Western soldiers were treated better than the Slavs and while they were sent to concentration camps, Slavs were put in the Jewish 'sector' which was much harsher with less food and worse punishments. The Westerners were rather prisoners instead of people who should be exterminated.

Was it common for the Nazis to send Western Prisoners of war to concentration camps like they often did to the Poles

As I mentioned, yes they were sent to concentration camps but were not in the harshest 'sectors' of these concentration camps. They were also sent to places like Colditz, which isn't necessarily a nice place to spend your time but it could be much much worse. Obviously, some PoWs got the short end of the stick, such as the members of Operation Frankton who were tortured hideously by the Gestapo for information. There's a good section on them in Special Forces Heroes by Michael Ashcroft.

Again, first time answering, I'm only 18 and by no means an expert but I hope it helps!

The racist mentalities of the Third Reich were a major component of the Third Reich's treatment of the Poles, but reducing the motivation for their actions to biological racialism obscures crucial aspects of why the would went to extremes in their occupation of Poland and what they were trying to accomplish when engaging in genocide. In short, the Third Reich did not conceive of the the land they occupied as "Poland," which they saw as a historical accident of geography. The Germans instead considered this geographical space as fundamentally German in origin and the clearing out of the Polish "steppe-culture" that had infested the land was a fundamental component of the regeneration of this Germanic space.

This conceptualization of Poland as German is apparent in the multiple documents produced by the SS and occupation authorities in Poland (the General-Government and the new Reichsgaue carved out of Polish territory). Much of this was highly utopian in nature and sought to create an ideal political and economic order for German settlement. Cities would reclaim their German character, the SS would lead a regeneration of the land and the new economy would be a harmonized form of capitalism in which the NSDAP mediated the relationship between labor and industry. In 1942 Konrad Meyer, the prominent agricultural scientist, SS lieutenant general, and Humboldt University chair for agricultural and farming policy, edited the book Landvolk im Werden. Published under the auspices of the SS’s Race and Resettlement Bureau (RuSHA), this massive anthology was an instruction manual for the Germanization of Poland. Meyer makes it clear that Germany was embarking upon a thorough reorganization of the Polish land for the exclusive benefit of its future German settlers. Within this vision, the newly Germanized cities had an unambiguous role according to Meyer. “What must therefore be sought in this planning,” he wrote, “is by no means a separation of the rural and urban, but the clear alliance with one another {emphasis original}.” He further reassures his readers that this new space will not see massive industrial conglomeration or an absurd “imitation of aimless Württemburgers’” disorganized microregionalism, but it will instead create a decentralized industrial and agricultural region inside the new German empire. Landvolk im Werden even produced a Heimat map which portrays the purported fundamental German nature of this space; the cities have German names and the map itself uses the symbolic language of Heimat (an expression of German cultural regionalism). RuSHA's sought to populate this region resettlement with both settlers from the Reich itself, but also Volksdeutsche settlers from the East whose Germanic character had not been polluted by modernity.

Of course, extolling the racial virtues of the Germans often meant denigrating the Poles. Utopianism and racial clearing of the enemey often worked in symbiosis. The editor of a prominent architectural magazine Deutsche Bauzeitung, H. F. Geiler ran a series of editorials in 1941 that also expressed the belief that the Eastern cities were following a clearly delineated path towards an organic city. This author’s analysis fused both racial and aesthetic judgments of German superiority by contending that:

the crooked and unsteady wooden picket fences, which are everywhere in the city, connect the makeshift cubic stone boxes of the Polish “colonists,” must be made to disappear before you can think of an actual reconstruction. In addition, a large part of most of the residential houses built by Jewish architects need to be demolished, not only because their style is an atrocity to healthy eyes, but also because they were close to collapse, even though they were only 10 to 15 years old. Most were brick sealed with lime and their supporting beams were often rotten, that they collapsed by merely pushing on them.

Similarly, Heinrich Friedrich Wiepking-Jürgensmann, who was Himmler’s special representative for the Race and Resettlement Bureau (RKfDV) would further delineate land resettlement as a racial war in his 1944 article “Hag und Heimat.” After outlining an RKfDV-led reconstruction of the forested Eberwald forest near Thorn, Wiepking-Jürgensmann would describe the new relationship of the landscape to the Eastern city. The Eastern city would constitute an alternative modernity compared to other world cities by virtue of the fact it was linked symbiotically to the peasantry and landscape. A resurgence of natural world under the tutelage of the regime was to the benefit of its new German settlers’ physical and mental health. “The villages and hamlets,” he clarified, “are framed and surrounded by a great green space, in which lies its protection of home, village, and garden.” Unlike in previous empires such as Rome or Ninevah, this conquered territory would not be a venue for racial mixing. “It need hardly necessary to point out [the results of intermixing in] the slums of White Chapel or the negro-quarter of New York,” he appallingly noted. Yet by observing this tenet of National Socialism, the children of these German settlers would be “worthy of partaking in the unending blessing of Mother Earth.” His article would include models of how the SS was combating soil erosion, which was the byproduct of racial inferiors' lack of a proper relationship to the soil.

This conceptualization of space imparts a certain form of perverted logic to genocide, as what it begs the question what should be done with the land's original inhabitants. While German occupation of Western Europe was exploitative, it lacked the grand utopian vision that characterized how the Third Reich's leaders conceptualized the East. Yet genocide was not an end onto itself for many of the leaders of the Third Reich, they were trying to construct a new order in which genocide was a necessary precondition.

Sources

Epstein, Catherine. Model Nazi: Arthur Greiser and the Occupation of Western Poland. Oxford: Oxford University Press, 2010.

Gutschow, Niels. Ordnungswahn: Architekten planen im “eingedeutschten Osten” 1939 – 1945.Gütersloh: Bertelsmann Fachzeitschr., 2001

Horwitz, Gordon J. Ghettostadt: Łódź and the Making of a Nazi City. Cambridge: Harvard University Press, 2008.

Uekötter, Frank. The Green and the Brown: A History of Conservation in Nazi Germany. Cambridge: Cambridge University Press, 2006.

Wolschke-Bulmahn, Joachim. “Violence as the Basis of National Socialist Planning in the ‘Annexed Eastern Areas,’” in Brüggemeier, 243-56.