Answer:
Many Europeans were already followers of communism or related socialist ideologies even before the war due to the impact of the achievements of the Soviet Union in the fields of proletarian emancipation, industrial modernization and social progress.
We can think of the incredible response of international communists/leftist volunteers to fight in Spain against Franco and his fascist supporters (among which we can even find Italians and Germans!!!).
It is true that many communist and leftist organizations received a cold shower after the announcement of the Molotov-Ribbentrop Pact between Soviet Union and Nazi Germany but many thought that was a clever, albeit twisted, move of Stalin to trick the Nazi…in some way!
Anyway, after WWII the Soviet Union came out as the winner and a winner that almost singlehandedly won the war!!!
At the end Stalin seemed not only the benign "Father" of the workers but also the brilliant strategist that won the war for the communist ideology...
So if the Soviet Union managed to do that its ideology had to be the winning one!!!
Any kind of doubt or opposition to Stalinist communism was forgotten or silenced.
Fascists/Nazis were responsible for widespread destruction, massacres, famine and murder while the communists looked at themselves as the defenders of humanity against the evil doers that on top of everything even created concentration camps and prosecuted minorities (thing that was already happening for years in Soviet Union anyway).
In many nations like France, Greece, Yugoslavia, Italy communist guerrilla resistant fighters against the Nazis were now acclaimed as liberators and redeemer of the motherland.
There was also opportunism…In my country, Italy, after the war it is said that many black shirts turned red….overnight…and not because of weird problem in washing them!
