Machiavelli's The Prince, Chapter III (part 2)
When a territory that has a different culture is conquered, luck and hard work are needed to pacify the population, and the best way to pacify the people is for the king to live in the new territory. When the king lives on his new land, he can find and rectify disorders quickly. However, if the king rules from far away, he is not informed of problems until it is too late to fix the problems. Additionally, the presence of the king prevents corrupt officials from pillaging the land. The population will be satisfied by the king's prompt reaction to injustices, and those who wish to be good have reason to love the king while those wishing to rebel will have more reason to fear the king. Because of this, the territory can easily repel an enemy's military.
Another option is to send subordinates to the conquered villages to rule in the kings name (a worse option is to send an army to control the province). Subordinates are inexpensive, and they aggravate only a few people when confiscating lands from peasant to build their own manor (similar to a mansion, estate, or plantation). The few offended people remain poor and unorganized, so they are not a threat to the king, while the rest are pacified, and do not threaten the king because they fear the nobleman may also confiscate their lands. Thus, subordinates are cheap, loyal, offend few people, prevent the offended from posing a threat, and discourage resistance to the king. That being said, the population must be either treated well or crushed, because a well-treated population is not prone to rebellion, and a massacred population does not have the power to resist, but people with slight injuries have a reason and the power to rebel.
The final option, keeping a military in the province, is the most expensive and will consume the king’s entire budget, in which case the conquered lands harm the kingdom rather than help it. A garrison must be manned by nobles, and these nobles must be replaced because they must manage their own provinces. As a result, the entire kingdom is faced with the difficulties of pacifying the territory, and the nobles may become hostile. For these reasons, a garrison is as useless as a colony is useful.
A king who controls a new territory different from his own should make himself the leader and defender of his neighbors, and should weaken his most powerful neighbors. This ensures that a foreign leader does not earn the respect of the new territory, in which case that leader would pose a great threat. Once a powerful king enters his neighbors land, he is respected by the provinces, which resent their own king. In this case, the provinces rally behind the invader, who profits with very little effort. The invader merely needs to prevent the provinces from gaining too much freedom, and he can easily suppress anyone who would oppose his authority. However, one who does not manage territories properly will soon lose those new territories, and he will have endless troubles managing them in the meantime.
All that follows are the examples of Rome and Louis XII of France. The examples show Machiavelli’s ideas in context, but I do not think they are critical to an understanding of The Prince, so I will not attempt to summarize them.