De jure is a Latin expression which literally means "of law". It is contrasted with de facto, which means "of fact". De jure ownership of land and titles is an important concept in the game as it determines AI behavior and the ability of the player to declare war.
Each duchy, kingdom and empire has a lawfully "right" land area in the game. If one ruler has occupied land that rightfully belongs to another, then they will have a de jure casus belli on that land area. For counties, the count holding the county title is the de jure liege of all holdings within the province. AI characters, including your vassals, will be upset with you if you hold land that de jure belongs to them—e.g., if they hold a Duke-level title and you hold one of their de jure counties. Each instance of this produces a negative opinion modifier which stacks without limit. Your own vassals will also be upset if you are the liege of vassals that de jure fall under their dominion—e.g., a baron in one of their counties calls you their liege instead of them. They will be pleased if you transfer these vassals to them. In general, all rulers in the game will believe they are fully entitled to whatever traditionally belongs to them and they will be willing to wage war to get their way if they think they can win.
The de jure ruler of a Kingdom/Empire also decides the Crown Authority of the realm. De facto owners of parts of a de jure realm are granted voting rights for laws if the rulers are of the same religion.
Titles with feudal elective succession have all their de jure vassal rulers of the same religion as nominators.
De jure drift
In general, a duchy will assimilate into a kingdom after being controlled by that kingdom for 100 years, and a kingdom will similarly assimilate into an empire in the same time frame. Every duchy in the game is considered to be part of a specific de jure kingdom. For example, in 1066, Flanders is part of the de jure kingdom of Frisia, but is entirely controlled by the King of France. If France controls Flanders for long enough, Flanders will eventually be considered part of the de jure territory of France. This will make the local rulers of Flanders regard the French king more favorably, and will also make it easier for France to reclaim the territory if it is conquered by another realm or declares independence.
A duchy will begin to drift into a kingdom when the following conditions are met:
- The entire duchy is within the realm of the king
- The king does not hold the crown of the current de jure kingdom to which the duchy belongs
- The ducal title either does not exist or is held by the king or by a vassal of the king
- The duchy is not part of the kingdom of Jerusalem
- The kingdom is not a tribal title
- The kingdom is its owner's primary title
After one year, drift is visible as dashed stripes the on kingdoms map mode. For example, the Duchy of Flanders may have its Frisian orange background marked by French blue stripes. Hovering over the duchy will show a tooltip indicating the number of years until full assimilation.
Whenever the conditions for assimilation are not met—for example, if the local ruler of Flanders rebels against France, or if one of the counties in Flanders is conquered by the Holy Roman Empire—the counter will tick backwards instead of forwards. When the conditions are again met—for example, if the duke's rebellion is put down - the counter will resume ticking forwards.
Kingdoms assimilate into empires in an identical fashion: the emperor must hold all territory in the de jure kingdom.
Assimilation allows titular kingdoms and empires to become non-titular and vice versa. If a de jure kingdom becomes titular, it can be created by whoever controls its de jure capital.
Assimilation can be both good and bad from a gameplay perspective:
- an assimilated Duke becomes more loyal, can vote in elections, and provide more levies
- but the assimilated Duchy no longer counts towards the total territories needed to claim a crown. For example, suppose as the King of Scotland you control the Duchy of Ulster in northern Ireland. Before the duchy assimilates, you control 4/13 counties in Ireland—you'd only need 3 more to proclaim yourself the Irish King. But if Ulster assimilates into Scotland, you'll now have 0/9 counties in Ireland and need to conquer 5 more to become King of Ireland.