The Paris Commune wins out in 1871 instead of being crushed by the French Third Republic, Ireland breaks away from British rule in the 1880s, and the Russian Revolution is just one of many similar revolutions in different countries. Finally Socialism also wins out in the rump U.S., following a revolutionary outbreak in Chicago.
Socialism works out as predicted by the German philosopher Karl Marx, bringing happiness and prosperity to all of humanity. (Marx himself is mentioned in the book as an enthusiastic supporter of the rebellious slaves, though he does not personally come to America to help them.)