Obedience to the rule of the daimyo is measured in public order. The factors that affect it are displayed on the province details panel, and include elements such as castle level, garrison size and the size of the tax burden placed on the populace. If unhappiness begins to outweigh happiness and repression at any point then the resultant unrest can lead to rebellion, if left unchecked. Keep taxes at reasonable levels, maintain sufficient food levels and ensure that castle towns are garrisoned to stop public order bubbling over into rioting or worse, full-scale rebellion.
Repression is the enforcement of your rule by the presence of garrison troops. The grey pips in the public order section of the province details panel represent repression through intimidation from garrisoned units. Whilst it is useful to keep the population in order, rebellion following a period of heavy repression can be difficult to suppress.
The overall happiness of a population is influenced by a variety of different contributing factors, including tax levels, food shortages, the presence of entertainment and religious buildings, and repression through garrison size. Rebellion becomes a risk when the people are so unhappy that repression can no longer contain public order.
When unhappiness becomes too great the peasants will riot. The best ways to stop a riot, and prevent it from escalating into a rebellion, are to either reduce taxes to appease the rioters or increase the garrison, which in turns increases repression and crushes the riot. Factors which may cause a riot are: low daimyo honour, food shortages, taxation, the presence of rampaging enemy armies in a province, and religious unrest.
There are two types of rebellion. A general whose loyalty has dropped too low may attempt to form his own clan rather than defect to another. If this happens, he will raise an army and try to capture the nearest castle town. The army will mostly consist of samurai with some ashigaru. Riots amongst the peasants can also escalate into full-scale rebellions, happening when a third of the unhappiness in a province is due to religious unrest. In the case of a Buddhist rebellion, the army will mostly consist of monks and ashigaru; a Christian rebellion will mostly consist of ashigaru with some samurai. The size of the rebel army is always linked to the level of the castle town in the province from which it has spawned.