The Soviet Union was quite literally built on slave labor (http://gulaghistory.org/nps/onlineexhibit/stalin/work.php). Arguably, the Gulag system of slavery was even more brutal than that of the American South. And like the South, they ultimately fell behind technologically and industrially to a free society.

What is forgotten today is that there were men who contemporaneously made the connection between communism and slavery while advocating for it, including George Fitzhugh:

http://en.m.wikipedia.org/wiki/George_Fitzhugh

Sociology for the South, or, the Failure of Free Society (1855) was George Fitzhugh's most powerful attack on the philosophical foundations of free society. In it, he took on not only Adam Smith, the foundational thinker of capitalism, but also John Locke, Thomas Jefferson, and the entire liberal tradition. He argued that free labor and free markets enriched the strong while crushing the weak. What society needed, he wrote, was slavery, not just for blacks, but for whites as well. "Slavery," he wrote, "is a form, and the very best form, of socialism."