In the subscriber-only THE FORGOTTEN MILLIONS at the Atlantic Monthly, Jonathan Rauch argues that "Communism is the deadliest fantasy in human history (but does anyone care?)." He links to The Victims of Communism Memorial Foundation, which points out

It is a great moral failing for a free society to misunderstand the extent of Communism’s atrocities. While the horrors of Nazism are well known, who knows that the Soviet Union murdered 20 million people? Who knows that China’s dictators have slaughtered an estimated 60 million? Who knows that the Communist holocaust has exacted a death toll surpassing that of all of the wars of the 20th century combined?

Hear, hear.