It's still a fair question to ask around WW2, given that there was already
plenty of evidence for atrocities perpetrated by the USSR. Indeed, it was noted at the time: Orwell wrote this in 1944:
"At this moment what is demanded by the prevailing orthodoxy is an uncritical admiration of Soviet Russia. Everyone knows this, nearly everyone acts on it. Any serious criticism of the Soviet régime, any disclosure of facts which the Soviet government would prefer to keep hidden, is next door to unprintable. ... The servility with which the greater part of the English intelligentsia have swallowed and repeated Russian propaganda from 1941 onwards would be quite astounding if it were not that they have behaved similarly on several earlier occasions. On one controversial issue after another the Russian viewpoint has been accepted without examination and then publicised with complete disregard to historical truth or intellectual decency. ... In the internal struggles in the various occupied countries, the British press has in almost all cases sided with the faction favoured by the Russians and libelled the opposing faction, sometimes suppressing material evidence in order to do so.
Throughout that time, criticism of the Soviet régime from the left could only obtain a hearing with difficulty. There was a huge output of anti-Russian literature, but nearly all of it was from the Conservative angle and manifestly dishonest, out of date and actuated by sordid motives. On the other side there was an equally huge and almost equally dishonest stream of pro-Russian propaganda, and what amounted to a boycott on anyone who tried to discuss all-important questions in a grown-up manner. You could, indeed, publish anti-Russian books, but to do so was to make sure of being ignored or misrepresented by nearly the whole of the highbrow press. Both publicly and privately you were warned that it was ‘not done’. What you said might possibly be true, but it was ‘inopportune’ and played into the hands of this or that reactionary interest. ... Events in Russia and events elsewhere were to be judged by different standards. The endless executions in the purges of 1936-8 were applauded by life-long opponents of capital punishment, and it was considered equally proper to publicise famines when they happened in India and to conceal them when they happened in the Ukraine."