Regions which it took itself.
We might also note that many regions the USSR took over it promptly deported or murdered a substantial number of people. For instance, over a hundred thousand were deported from the Baltic States in 1940, many of whom were never heard of again. (Even more after the USSR reasserted control 1944-1945). It's assumed they were shipped to gulags and died there, largely unrecorded. Although maybe plenty were just slaughtered and buried,
because the USSR also did that sort of thing. In terms of Ukraine, of course, it also famously managed to starve millions to death. Doesn't make the regime popular.
It is thus perhaps no surprise that many areas dominated by the USSR viewed the Nazis as potential liberators. As abhorrent as the Nazis were, for some nations they appeared the lesser evil and only realistic route out from Soviet oppression.
Russia ruthlessly manipulates this in the modern day to assert its moral and territorial ambitions by linking resistance to Russia as Naziism. We can and should see through by recognising just how appalling the Soviets could be. We don't need to condone Naziism to understand why opponents to Soviet rule would ally with them to escape the Soviets.