Long story short: In 1956 Hungary, which just a decade prior was a member of the Axis, sparked a counter-revolution to overthrow the Hungarian People's Republic. The Soviet Union under Premier Khrushchev mobilized the Red Army and crushed the counter-revolution. This was shortly after Khrushchev's "Secret" speech that heaped trash on the grave of his predecessor and gave the imperialists free-reign to smear and tarnish the Reds at every opportunity with little to no pushback that continues to this day.

Obviously when you crush a counter-revolutionary movement to restore bourgeoise "democracy", the international bourgeoise use it as an opportunity to further smear your good name. The Western Left, still reeling from Khrushchev's bullshit speech, were further alienated from the Eastern Left due to the perception of the "Barbaric" Reds slaughtering innocent Hungarian protesters that simply wanted to achieve freedom and liberty and break free from the Iron curtain. Communists in the West that upheld the decision of the Soviet's leadership to put down the fascist counter-revolution were attacked wholesale, with the CPGB members that upheld the decision were publically taunted as "Tankies" for supporting the brutal Red Army tanks that ground down the last hope for Hungarian freedom with their treads.

Longer reading: The Truth about Hungary By Herbert Aptheker. Published in 1947, one year after the Hungarian Revolution, the book explores the historical and material conditions of Hungary leading up to the '56 revolution, the event itself, and the response of the international socialist and communist movement of the time period.