Any city that ends a turn with a happiness of 68 or less will revolt. Revolting cities break completely away from their parent Civilization, establishing a new government with a fiery hatred of their former oppressors. In addition, there is a chance that the revolutionary passion will spread to nearby cities. Any city near to a revolting city has a chance of revolting itself. The probability of the revolt spreading successfully depends on the happiness of the nearby cities. Military in and around a revolting city will switch allegiance and join the new revolutionary government.