Communist rule was a disaster. No question. But Michael Kazin points out a paradox: In many Western countries, communist movements often did a great deal to advance liberalism in the 20th century. In France and Italy, many communist voters wanted no part of Stalinism, and instead provided crucial support for the creation of the social-democratic state. In the United States, meanwhile, the communist left helped lead the push for racial equality in the 1940s and 1950s and spurred the growth of labor organizations such as the CIO in the 1930s:

The Popular Front allowed rank-and-file Communists–most of whom were either immigrants or had grown up in ethnic or racial enclaves separate from the larger society–to embrace Americanism. The reformist party line gave CP members permission to follow their hearts as well as their minds–to identify and work with a variety of their fellow citizens in big battles for undeniably important and feasible ends. In 1937, the Young Communist League chided the Daughters of the American Revolution (DAR) for neglecting to celebrate the anniversary of Paul Revere’s ride. They marched up Broadway in New York City with a sign that read, "The DAR Forgets but the YCL remembers." It didn’t seem absurd at the time.

Communists thus put grassroots muscle, and their tightly blinkered idealism, behind the goals of the New Deal and joined the coalition that kept it in power. "I don’t turn my organizers upside down to see what kind of literature they have in their pockets," CIO leader John L. Lewis told critics who doubted the wisdom of allowing Leninists to spearhead union drives in auto factories and steel plants. In fact, that literature often promoted causes like interracial unions, progressive taxation, and national health insurance that many Democrats in Congress opposed.

Obviously this doesn't "excuse," say, the Cultural Revolution, but there's no reason it needs to. Of course, once the Cold War began, mainstream liberals distanced themselves furiously from their erstwhile communist allies, while adopting many of their positions--on race, for instance. Kazin argues that liberals were right to do so (not all those allegations about espionage were false). Fair enough, although I'd say that, in hindsight, the AFL-CIO's long decline began when it purged itself of leftists in the 1950s and became a conservative group bent on destroying progressive unions around the world. More on that here. Not everyone agrees. Either way, Kazin's essay is well worth reading.