A Civil War is a game mechanic which splits a large civilization into two different nations if its capital city is captured. The original Civilization remains at war with the conquering Civilization but new Civilization has a neutral attitude towards the conquering Civilization.
Civil War conditions
A civilization may go into Civil War if its capital falls and:
- Conquered by a Civilization smaller than it
- Must have at least 6 cities
- Must have a capital city
- Must have high corruption rate in many of its cities
- A different "color" is available. If the player chooses 7 civilizations and no Civilizations have fallen than Civil Wars are not possible
- The capital is conquered before a certain date?
References
https://forums.civfanatics.com/threads/civil-war-in-civ1.347072/