Once you reach the Modern Age in you'll have the choice to focus on three distinct ideologies: Democracy, Fascism, and Communism.
Each has their unique benefits, and choosing a particular Ideology will have implications with other leaders in the campaign.
Your choice of Ideology can depend on a number of factors and will largely be based on what strategy or Legacy Paths you plan on pursuing toward an eventual victory.
Each Ideology unlocks certain effects and social policies that are catered around two particular resources, and has a total of three Civics associated with it.
To unlock the Ideology Civic Trees, you first have to unlock the initial Modern Age Civics, Social Question, Modernization, and Natural History, then complete the Political Theory Civic.