Complex sentence: Ideas of Socialism had been gaining momentum among working class citizens of the world since the 19th century, these culminated in the early 20th century when several countries and subsequent nations formed their own Communist Parties.
Split into simpler sentences: Ideas of Socialism had been gaining momentum among working class citizens of the world since the 19th century. These culminated in the early 20th century when several countries and subsequent nations formed their own Communist Parties.