Europa Universalis 4 Wiki
A personal union is a form of diplomatic relationship where one monarch rules over two nations. It can be formed by having a royal marriage with a country whose monarch dies without an heir (usually of the same dynasty). It may also be formed by enforcing a claim through war or, in certain nations, through an event or decision. Once formed, it works similar to vassalage: the nation leading the union (the senior partner) gains the throne and puppets the nation who lost their monarch (the junior partner). Personal unions can only be formed over Christian monarchies (Catholic, Hussite, Protestant, Reformed, Anglican, Orthodox, and Coptic).
Unlike other subject types, the junior partner in a personal union can have their own subjects. For example, forcing a union over Bohemia as Austria will not end their overlordship over their vassals in Silesia.
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Ansbach over 
Bayreuth.
Aragon over 
Naples.
Burgundy over 
Brabant, 
Holland and 
Flanders.
Denmark over 
Sweden and 
Norway.
Hungary over 
Croatia.
Provence over 
Lorraine.
Saxony over 
Thuringia.|
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Please help with verifying or updating this section. It was last verified for version 1.30. |
There are several ways in which a player can form a personal union, or fall subject to one.
Several countries in this game have the opportunity to form a personal union with another country through an event. All events resulting in a personal union can only fire if the junior partner is AI controlled (e.g. if the player chooses Aragon, the “Iberian Wedding” event will make Castile the junior partner).
For two countries to form a personal union diplomatically the following prerequisites need to be met:
If the ruler of the target country then dies heirless, and there are countries that can become a potential senior partner to the target country, one of the following happens:
Which one of these happens depends one the phase/tier in which the target country is
Take note that claiming a throne causes a −100 opinion malus with the target country, and a −50 opinion malus with all countries that the nation claiming the throne has a royal marriage with but gives some priority to being selected as the default senior partner for the union (see rules below).
If there are multiple potential senior partners, then the following rules are used to decide the "best claimant":
When a country falls into a personal union under another country in such a manner, the game will identify another country among all countries that were considered as a potential senior partner, have rivaled the new senior partner, or are rivalled by the junior partner or have a royal marriage with the new senior partner. They will receive an event where they can either accept or decline to enter a succession war over the new junior partner. If no country is willing to contest the personal union, the target country will fall under a personal union under the new senior partner peacefully. If a succession war ensues, the attacker, if successful, can enforce a union over the junior partner, or demand that the senior partner release the target country. Note that the 'defender' of the war will have the junior partner on their side, as the game considers the war to dispute the union that has already formed.
Non-monarchies can contest a succession war if they have a government reform which allows royal marriages.
The player, in order to maximize their chances of becoming the senior partner in a personal union, will need to get their dynasty on as many countries' thrones as possible. A target country can adopt the player's dynasty if the target country's king dies without an heir or the target country is in an interregnum; the target country will not fall under a personal union when the king dies, the target country has a high opinion of the player, and the player has higher prestige than the target country. The chances increase if the player has more than 4 royal marriages. The player can check this at any time by hovering over the target country's ruler's name; it says "A noble from X succeeds to the throne" if that is the case.
In order to further increase the chances of the player's dynasty spreading, they can target kingdoms with old rulers and no heirs specifically, ideally those that have few royal marriages. The player can check this at any time through the "Disputed Succession" alert.
The player can check at any time if a target country is at danger of falling under a personal union by hovering over that country's king's name in the diplomatic view and checking the text after "On Monarch Death:". Countries that have no heir are listed in the "Disputed Succession" alert with a yellow font; such countries with the player's dynasty or a royal marriage are listed in green font. The player will receive a red alert themselves if they are in danger of becoming the junior partner in a personal union.
A country can form a personal union by attacking another country using either of three casus belli: Force Union, Claim on Throne, and Restoration of Union.
The Claim on Throne CB can be acquired by taking the Claim Throne diplomatic action if the target country shares the same dynasty, has either no heir or an heir with a weak claim, and has a royal marriage with the country. If the country has acquired the CB in this way, the claim will expire if the target country obtains an heir with a strong or average claim, or the royal marriage used to obtain the CB ends for any reason. (Note that dissolving an alliance will cause a 5-year truce, and, as you cannot cancel the royal marriage without losing the CB; declaring using the CB will cause the loss of at least 
1 stability).
The Restoration of Union CB is acquired by senior partners of a personal union if the junior partner breaks free (cf. #Ending a personal union) or through a country's mission tree (see below). A country can also acquire a Restoration of Union CB against a monarchy with which it has a royal marriage if that monarchy introduces a new heir.
In order to enforce the claim, the player needs to attack the target country using any of the casus belli above. While the target country is able to call in all its allies as this is a defensive war for them, some or all of the attacker's allies may be unwilling to join due to the opinion modifier "Does not want X in a personal union under Y". The war goal with either CB is to occupy the target country's capital, and enforcing the union costs 84% (claim throne) / 60% (restoration of union) war score, regardless of size. This will cause 
aggressive expansion relative to the target country's size. Enforcing the union will also cause a −100 opinion modifier with the new subject (which is on top of the potential −100 opinion modifier caused by claiming their throne).
Golden Century DLC. Golden Century DLC. Golden Century DLC. Only available to Spain. Golden Century DLC.
Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC. Emperor DLC.
Lions of the North DLC. Lions of the North DLC.As with all subjects, it is possible to interact with a junior partner. This is done through the subject interaction section of the subject tab of the country interface.
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A personal union is similar to vassalage. The senior partner controls the diplomacy of the junior partner, the junior partner will always join their wars and will always occupy provinces for the senior partner, as long as their liberty desire doesn't rise above 50%. The junior partner cannot declare war, negotiate separate peace treaties or enter royal marriages. As in vassalage, being the senior partner of an elector in the Holy Roman Empire will result in a +50 point bonus to their electoral vote and a −50 point malus from all non-controlled electors.
However, unlike a vassal, the senior partner does not receive any income from lesser partners in a personal union.
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A personal union can end in any of the following ways:
A junior partner in a personal union can be diplomatically integrated by the senior partner through the Integrate option. Integration occupies a 
diplomat until complete, and results in direct control of the former junior partner's territory and military, while also gaining control of its subject nations.
The requirements for initiating integration of a junior partner in a personal union are:
capital.Integration progress of junior partners appears in the diplomacy interface as a small vertical progress bar next to the personal union icon 
. If the integration process is cancelled before completion, all progress is lost, including the invested 
diplomatic power. Diplomatic integration begins from 0 progress at this point. Unlike 
vassal annexation, personal union integration can be cancelled if the personal union ends.
Integration will be paused if:
capital.Integration will be immediately cancelled at the end of the month, if both countries are no longer members of the same religious group. This can be avoided by force-converting the Junior partner or changing the Senior partner's religion through rebels.
The total cost of integration is 8 diplomatic power per 
development of the junior partner. This cost is increased by the 
hostile core-creation cost modifiers of the junior partner's corresponding provinces, and decreased by 
administrative efficiency and modifiers that reduce 
annexation cost. These modifiers are displayed below:
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Traditions | Ideas | Bonuses | Policies |
|---|---|---|---|---|
| In patch 1.37 no ideas and policies have the modifier “hostile core-creation cost on us” | ||||
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Traditions | Ideas | Bonuses | Policies |
|---|---|---|---|---|
| +5% |
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— |
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Traditions | Ideas | Bonuses | Policies |
|---|---|---|---|---|
| −25% | — |
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— | — |
| −15% | — |
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| −10% | — |
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— |
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The full formula for diplomatic integration cost is:
The senior partner invests diplomatic power into the integration process each month at the following rate:
culture group
diplomatic reputation.If the total is 0 or negative, integration will pause, but not reverse.
Completing diplomatic integration of a junior partner results in the following effects:
diplomatic reputation.vassals, 
colonial nations, and 
protectorates.
local autonomy in all new cores (or their previous autonomy level, if above), and 0 if the senior partner already had a core.Inheritance instantly annexes the junior partner with no autonomy. Every time the ruler dies or abdicated, the junior partner(s) has a chance to be inherited. It can't happen if the union hasn't been stable for at least 50 years.
The probability of inheritance is:
StabilityDiplomatic reputationculture groupIf the junior partner has too many provinces, the chance of inheriting it can be below 0% (meaning it will never be inherited). Such countries can only be integrated, or in some cases a special decision can be used to inherit the country.
If the senior partner of the union is in the HRE then inheriting an elector of the HRE gives the electorate to the senior partner.
If the senior partner's ruler dies while the junior partner has a negative opinion of its overlord, the junior partner will become an independent nation again. Personal union is not ended when the regency is over and the heir becomes a ruler. The senior partner will get a "Restoration of Union" casus belli. A junior partner can also declare war on their senior partner at any time - even while the nations are in a war together - to gain their independence. If their liberty desire remains low, then they will never declare independence. Once the senior partner has a high enough development and strong enough army versus their personal union subject, they will never declare independence on their own, even at high liberty desire.
Note that 
disloyal or 
rebellious subjects can accept support independence offers from other countries (typically the senior partner's enemies), which will increase their liberty desire significantly and cause them to declare a war for their independence, where the goal is for the attacker to defend its capital.
Should the senior partner be subjugated or annexed in a war, the junior partner will become independent.
| Diplomacy | Diplomacy • Diplomatic feedback • Envoy • Espionage |
| Other | Defender of the Faith • Great power • Hegemon • Prestige • Regions |
| Political structures | Emperor of China • Holy Roman Empire • Papacy |
| Relations | Personal union • Relations • Subject nation |
| Concepts | Corruption • Governing capacity • Overextension • Power projection • Rebellion • Regions • Stability • States and territories |
| Court | Advisors • Consort • Monarch power • National focus • Ruler • Ruler personalities |
| Estates and Factions | Factions • Estates (Base estates (Burghers • Clergy • Nobility) • Cossacks estates (Cossacks • Dhimmi • Tribes) • Dharma estates (Brahmins • Jains • Marathas • Rajputs • Vaishyas) • Domination estates (Janissaries • Eunuchs)) |
| Events and Missions | Decisions • Disasters • Events • List of decisions • Missions |
| Goverment | Absolutism • Culture • Government • Government rank • Modifiers • Policies |
| Province mechanics | Autonomy • Buildings • Canal • Capital • Core • Province |
| Religions | Christian denominations • Eastern denominations • Muslim denominations • Other denominations • Pagan denominations • Religion |
| Specific governments | Native council • Parliament • Steppe hordes |
| Colonisation | Exploration • Colonisation • Colonial nation • Tariffs • Trade company |
| Economy | Debase currency • Development • Economy • Privateering • Production • Raid coasts • Tax |
| Trade | Trade • Trade company • Trade goods • Trade nodes |
| Customization | Nation designer • Random New World |
| Other | Achievements • Luck • Score system |
| Ideas and Policies | Idea groups • National ideas • Policies |
| Ages and Institutions | Ages • Institutions |
| Innovativeness and Technology | Innovativeness • Technology |
| Declaring war | Alliance • Casus belli • Claim • Peace • War exhaustion • Warfare |
| Defense | Fort • Zone of control |
| Land warfare | Army • Condottieri • Discipline • Drilling • Land units • Land warfare • Manpower • Militarisation • Mercenaries • Professionalism |
| Naval warfare | Flagship • Naval blockade • Naval doctrine • Naval units • Naval warfare • Navy • Sailors |
| Other | Force limit • Military leader • Military tradition |
![\text{dip point cost} = \text{floor}\left( \sum_\text{per province}\left[ 8 \cdot \text{development} \cdot (1 - \text{administrative efficiency}) \cdot \left(1 + \sum\text{annexation cost modifiers}\right) \cdot \left(1 + \sum\text{local coring cost modifiers}\right) \right] \right)](https://en.wikipedia.org/api/rest_v1/media/math/render/svg/719dd9298cba8e00846ab114cb370a21d63bb863)