In the early years of the 20th century, a spectre was haunting Europe. The forces of victorian capitalism, and industrialization resulted in a lot of wealth, but for very few people. Many worked very long hours, in dangerous conditions. They produced giant amounts of wealth that only went to their bosses.
Enter the mid-19th century philosopher Karl Marx. He wrote that this increased inequality would not be able to sustain itself, and that at some point in the future, workers of the world would overthrow their rulers, and implement a new world order. One without inequality between anyone. A state called communism.
By the First World War, many activists across the world were seeing this battle between distant monarchs as the ultimate sign that it was time for the world order to change. In the age of nationalism, they believed in creating governments working to make the world into that communist dream Marx wrote of. Governments moving in this direction are the dictionary definition of socialists. 
The idea was to create a Communist World Revolution, overthrowing capitalism all over the planet, and organizing a truly global working class against the bosses, kings, and presidents that ruled over them for too long. This is the story of that revolution.