One argument that conservatives like Dennis Prager use to elicit support for Trump in the general election is to claim that it's analogous to an alliance with Stalin to defeat Hitler. There are, however, some basic problems with that comparison:
i) Historical comparisons tend to be very loose analogies. Often, past and present circumstances are quite different.
ii) More to the point, the comparison is relative to your viewpoint. If, during WWII, you were an Englishmen, or a Jew, or a member of the French Resistance, or a member of the Italian Resistance, then defeating Hitler is your number one priority. He poses a direct and urgent threat. An existential threat to your national survival or ethnic survival.
Empowering Stalin is very dangerous, but that's a battle for another day. If you don't beat Hitler, you won't survive to fight another day. So that comes first.
iii) If, however, you're a Russian, Central European, or Eastern European, then you might assess the relative threat very differently, especially if you happen to be prescient. It's a choice between different sets of victims. There's some overlap, but Russians, Central and Eastern Europeans had more to lose from Stalin than Western Europeans.
And even if Hitler won, Germany would be a severely overextended empire. Given how many young German men died in the war, how could Germany muster an effective occupation force to police its empire–assuming it won?
My point is simply that the comparison between Hitler and Stalin is provincial. In the nature of the case, it's more appealing to prospective victims of Hitler than prospective victims of Stalin.
The point of the comparison is to illustrate the choice between bad and worse. But worse for whom? For many Russians and Eastern bloc countries, Stalin was a worst-case scenario. Was the Soviet Empire better for them than a Third Reich?