A civil war can be started at Level 10
The costs is 100 gold plus 10% of the population of the country.
you need 33.3% of participants for a war to successfully begin.
If the loyalists win, nothing happens. However if he rebellion wins the rebel leader becomes president and congress members will be replaced at random with the top hitters on the rebel side according to the rebel dominance in the battle. For example, if the rebels did 65% of the damage in the battle, 65% of congress seats will be replaced with top rebels.
Once Started it is like a normal Subsidies/Resistance war with the first to win 8 rounds wins the war