I often boldly state, as an iron law of history, that you don't get civil war in a democracy. Putting it more cautiously, I just think that civil war is less likely in a democracy.
The counterexample that most people come up with is the American Civil War. In February 1861 the Confederacy withdrew from the Union, and then war broke out in April 1861. So I'd argue that the American Civil War was a war between two democracies, not a civil war within a democracy.
Another counterexample that's raised is the Spanish Civil War. I concede that technically this really was a civil war within a democracy. However, I point out that Spain had been a democracy for only 8 years before civil war broke out, and so democracy hadn't properly bedded in.
Even academics agree!
I live in Europe, and we've had continual civil wars here over the centuries. World War I and World War II were European civil wars that spilled over into the rest of the world. When I say, 'European civil wars', I mean wars within Europe, not wars within the individual states of Europe.
With the formation of the EU we finally had a democratic Europe, and so the chances of further civil wars were significantly diminished. One of the reasons I am disappointed by Brexit is that it reduces the size of the EU, and so the chances of war increase. In the same way that the Confederacy left the American Union, the UK left the European Union. Both sides are still democracies, but the whole is not a democracy. I've tried to put a number on this way of looking at democracy, and I call it the Democratic Magnitude.
In short, the higher one's Democratic Magnitude (DM), the less likely war is. Before we left the EU the DM of a UK citizen was 6.42%, but now that we've left the EU it is 0.81%.