To what extent is there an accepted practice of military conduct among the nations of the Inner Sea? Obviously nations like Cheliax and Katapesh engage in slaving and the Hold of Belkzen often raids beyond its borders, but among the neutral to good countries, is there any consensus on a law of war?
I ask because I've been running Kingmaker for my group and my players are beginning to inquire into the state of world politics and how a new kingdom ought to behave. Particularly, I couldn't find anything on what common behavior is after a city or fortress has been captured or how most armies are paid. In our own history, most armies were a form of levy that were partly recompensed through the right to loot, both while on campaign and when they captured a city. I'd like to know what they're in for, should their ambitions draw their attention abroad.
In River Kingdoms I think that would be like in late-medieval or renaissance Europe. Depending on a kingdom, they may use a mercenary system or a feudal system, or a mixture of both.
In the mercenary system the soldiers are paid regularily with possible bonuses for victories, and they are being supplied by whoever hired them.
In the feudal system, men fight for their lord because of the land they were granted beforehand, and they want to keep the right to use that land. So king has to grant land to his barons, barons to knights, and knights to peasants. When going to war they are supposed to bring their own equipment, and usually supplies.
In the mixed system, some barons are exempted from the duty to provide soldiers, but instead they need to provide money and supplies, which are used to hire mercenaries. The king also hires mercenaries ussing his own coffers.
The right to loot the enemy defeated in field would be considered obvious (it was common practice for knights to capture horses to replace the ones they might have lost, and to hold opponents for ransom).
Looting the countryside can be used to reduce costs of supplying the army, but it disperses the forces, and less of them are ready for the battle when it comes. Because of that the leader wouldn't normally allow for independent looting, although if he cannot supply his forces correctly, they are going to resort to it anyway.
Whether the pillaging of a city would be allowed depends on the strategic objectives of the leader. It can be used as a way to convince other cities to surrender without a fight, but generally a king wouldn't want to pillage a city he hopes to incorporate into his domain, since prosperous cities were an important source of income.
The pillaging might happen whether the leader likes it or not, if his troops are undisciplined, underpaid or angry, or if they simply don't care what said leader thinks.
Ultimately, your PCs [try to] determine how their country and its armies behave, and their reputation will go before them. If they start out ransacking everything they see, people will expect that. If they play nice, people will expect that. And so on. And their neighbours will be different - some nice, some nasty, some unpredictable.
Granted, but what I'm trying to determine is how common such outbursts of violence are. Their kingdom is (ostensibly) neutral good in alignment. I recognize the weaknesses in using such a system, but it does bear a handy indicator for the social norms, if not what will occur every time. I fully expect that they will lose control of their troops at some point, but in such a case would the militias and guardsmen of such a society find it normal or expected to be paid by looting the cities and countryside of their enemies? What about a more neutral one? Such behavior was very much the case in our own history, even when armies were fighting on behalf of relatively benign or righteous causes. Outside of the threat that such an action would bring, I'm not sure if the River Kingdoms would view the sack of Pitax as an atrocity or simply the normal behavior of a conquering army.
If the garrisons aren't supplied properly, they won't go attack the enemies of their employer (at least not right away). At first, they may turn into bandits and brigands that would ransack the land they were previously guarding. If they are faced with opposition sent by the king and decide that it's easier to perform banditry in another country, they may move and become a problem for the neighboring ruler, but it's unlikely they will become a force strong enough to pillage his cities.
Alternatively, they may strike a deal with a neighboring ruler that they will let him annex the lands they were guarding if he pays them.
Militias, recruited from the local population, don't require much supplies as they are fed by their own community, but if the ruler is too harsh or doesn't fullfill his duties, the whole region can simply denounce the obedience. They may turn to another kingdom for protection.
Sacking a city is something that can only be done by a large army, which means there has to be a war going on and a ruler has gathered that army. Pitax is considered one of the more stable countries in River Kingdoms, and Neutral Good ruler wouldn't start a war with it unless there was no other way to stop some great evil.
If an army sacks a city because it's unpaid and undisciplined, it would make the leader less respected, but not hated or feared. He would come out as someone whom even his own subjects do not obey, his kingdom may be painted as an easy target. If you are attributing alignments to kingdoms, something like that would be a strongly Chaotic Neutral act for the kingdom (not the ruler).
If the leader orders the pillaging beacause the city offended him in some way, and wants to make a show of force, that could create some fear. Not necessarily hated, as this can be seen as a natural consequence of the war. For example: they city inovked the ruler's wrath, didn't apologize, so he came with his army, they refused to surrender, he pillaged the city. Lawful Evil.
If the leader lets his troops to pillage a city because of his own bloodlust, or because of the demands of his soldiers that would cause a lot of hatred, and can be anough of a reason for other countries to go to war with this new wannabe-conqueror. Chaotic Evil.