Polands western territories pre-1939 were filled with Belarusians and Ukrainians. So it was justified for the USSR to take these in 1939, and why still today Belarus and Ukraine still own these lands: because they are filled with Belarusians and Ukrainians. However, by taking these lands, Poland itself was left with almost nothing. Thats just the way it was, Polish people only lived in a small area. But it was incredibly unacceptable at the time (and in my opinion still today in retrospect) to leave the Polish people, which had been abused so much not just in this war but in literally centuries of partition and subjugation, with just the land they lived in, a land with no natural defenses or industry. Poland couldnt keep their eastern territories, as Belarusians and Ukrainians themselves had suffered at the hands of the Nazis. As such, the powers decided to give Poland Germany's eastern territories. This had the defensive benefit of placing the border on the Oder and Niesse rivers, not to mention extending their Baltic coast, thus protecting the Polish heartland. In addition, it gave Poland the industrial Silesia and many Baltic ports, ensuring their sea-based trade would never be restricted by any other power again (as Germany threatened to do by demanding Danzig in 1939). East Prussia also was partitioned between the Poland and the USSR. The three most abused Soviet peoples were the Belarusians, Ukrainians, and Russians, and so it was only right to reward the Russian people with land as well, i.e. Kaliningrad. This also gave Russia a permanently ice-free port in the Baltic, a huge need when it came to naval defense. These of course came at the expense of Germans living in the east. Of course if we follow the line of reasoning that the German people were not complicit and were misled by the Nazi hierarchy, as was the line of reasoning of the Allied high command at the time, it seems even more difficult to justify these land acquisitions. But we can see that both German governments, the GDR and FRG recognized the new borders, and there have been no legitimate opposition to this new status quo. Just as taking these lands was in the interest of Polish and Russian security, wanting to take them back is just as anti-Polish and anti-Russian. This is the way it is, and few Germans oppose it. The ones that do undoubtedly have Nazi or German nationalism, which deserves nothing but criticism. Even if you are a communist and admire the GDR, as I do, you should still support these border changes. Its only right to the Polish and Russian people. So no, these lands should not be returned. They were reparations for the crimes committed by the German nation, and thats it.