"Liberals spent the Cold War refusing to see communism for what it was." Robert Conquest on 'how the mind of the liberal became so much a subject of self-deception.'"
A liberal is, by definition, one whose aim is the furtherance of ever greater political liberty, freedom of thought, and social justice. A number of those who thought of themselves as, and were thought of as, liberals became apologists for Stalinist or similar regimes whose most notable characteristics were extreme terror, narrow dogmatism, social oppression, and economic failure. That is, they were all that the liberal tradition opposed. How, and why, did a number of liberals explicitly, and a large swath of liberaldom implicitly, overcome this objection? How did this apparent paradox come to pass? Why in the 1930s and later do we find a sort of general infection of the atmosphere in which much of the intelligentsia moved? Even apart from those who became more or less addicted to communism, there was also a stratum that usually gave the Soviet Union and such regimes some moral advantage over the West.
First, of course, we should say that there were many liberals--and in general many on the left--who kept their principles unsullied and were often among the strongest opponents of the communist despotisms. Liberal is, indeed, a vague term. Many of us would take a "liberal" position on some issues, a "conservative" one on others--as most of the American or British people in fact do (an attitude shared by the present writer).
These two vaguely differentiated attitudes are the poles within the normal development, or balance, of a civic or consensual society. But all those with a reasonably critical intelligence, whether "conservative" or "liberal" on other issues, were hostile to the USSR. Those who supported it unreservedly were Communists; those who excused it may have thought of themselves as liberals, but to that extent they degraded the term.
Read this entire article on the Hoover Institute website.