How Was Cuba and China "Lost" to Communism?

Communism is a political and economical system that was once one of the most powerful forces in the world. As late as 2004, only Cuba and China maintained a communist form of government. How were they lost to communism? Let's start with a brief history of communism.
Karl Marx and Friedrich Engels, both economic philosophers, believed in communism. They believed that violent revolutions were sure to happen in the natural course of history. Marx wrote a book called the Communist Manifesto. They believed that socialism was an intermediate stage where some class differences remained and communism was a final stage of society where there were no class differences. …