In early forms of democracy, only people with resources (land, wealth) had the vote. A war is easily squashed if the revolutionary side has no resources, so it suffices to keep happy only those with resources. Later, as the industrial revolution put significant resources in the hands of former peasants, everyone was given the vote.
The constitution of the USA gives everyone the right to bear arms. This assures that the vote will not be denied anyone, because everyone could potentially start an uprising.
Also, note that democracies do not elect a leader, they elect a set of people (reflecting a range of interests) who then work together to run the country.
What voting system minimizes the chance of civil war?
We want to minimize the number of revolutionaries. We could ask people under what circumstances they might become a revolutionary, then find a set of people who minimize the number of potential revolutionaries. Typically, a person might become a revolutionary either if no one they approve of is elected or if someone they intensely disapprove of is elected.
This seems to indicate a variation on approval voting: From the list of candidates, a voter nominates a set of people they approve of, and set of people they disapprove of. The vote indicates that a person will be upset if either none of their approved candidates is elected, or one or more of their disapproved candidates is elected.
For example: At the next Australian election I might vote approval for Democrat/Green candidates, disapproval of Liberals, and express no opinion about Labor.
Once votes are collected, the set of candidates that minimizes the number of upset voters is chosen. (note: the set may be of arbitrary size)
This could be used to vote for parliamentarians of a country, the council of an organization, or even policies of an organization. It could also (if I get off my ass) be used instead of my little approval vote thingy in the left side-bar.