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.
- 1 Countries starting with a personal union in 1444
- 2 Forming a personal union
- 3 Subject interaction
- 4 Effect
- 5 Ending a personal union
- 6 Achievements
Countries starting with a personal union in 1444
- 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.
Forming a personal union
There are several ways in which a player can form a personal union, or fall subject to one.
Personal union by event
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 place Castile as the junior partner).
- Poland can form a Personal Union over Lithuania through its Successor of Wladyslaw III event. This will result in Lithuania forming a Personal Union under Poland, and later enables the Polish decision to Form the Polish-Lithuanian Commonwealth.
- Castile can form a Personal Union over Aragon through its Iberian Wedding event. This will also make Castile the senior partner in a Personal Union over Naples, if they are in a Personal Union under Aragon at the time that the event fires. This enables Castile to later Form the Spanish Nation Diplomatically.
- Aragon can also form a Personal Union over Castile through the Aragonese Iberian Wedding Event if Aragon is controlled by a player. Aragon can later take a decision to inherit Castile and form Spain.
- Aragon can additionally form a Personal Union over Navarra if their 1444 heir, Joan II, ascends to the throne while he still rules in Navarra. (They are the same person.)
- Spain can form a Personal Union over Portugal through the Portuguese Crown Event between 1550 and 1650, if Spain has a royal marriage with Portugal and the Portuguese opinion of Spain is at least 150. This event is very rare (MTTH is 2000 months).
- Naples can receive an event that allows it to be in a Personal Union under France.
- Brandenburg can get a personal union over Prussia if Prussia is a vassal of Brandenburg or if Prussia's opinion of Brandenburg is at least +100. This event can still fire even if Prussia is a vassal of another country.
- Brandenburg can form a union over Ansbach if their initial 1444 heir Albrecht Achilles ascends the throne while the 1444 Ansbach monarch Albrecht Achilles II still reigns. (They are the same person)
- Bohemia can get a Personal Union over Brandenburg and Saxony via event after vassalizing them and completing the according missions. The necessary subjugation CBs are also provided by missions.
- Any Christian country can get an event that can lead to a peaceful union with another Christian nation if both rulers are between 16 and 40 and the two nations involved neighbor each other, have a high enough opinion of each other, have no heirs, both rulers are of a different sex and are either of the same dynasty or in the same culture group. This event is quite rare (MTTH is 500 months)(cf. Dynastic events#Event chain: A Political Marriage).
Personal union through diplomatic means
For two countries to form a Personal Union diplomatically the following prerequisites need to be met:
- The potential senior partner either
- share the same dynasty with the target country (potential junior partner), or
- has a royal marriage with the target country, or
- has claimed the throne of the target country
- The target country has no heir
- The target country is not already a senior partner in a Personal Union
- The target country is at peace
If the ruler of the target country then dies heirless, the target country is at peace, and there are countries that can become a potential senior partner to the target country, the target country will fall under a personal union. 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 player 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":
- There are three brackets of claimant countries. In order of highest priority to lowest they are:
- Countries that have used the claim throne diplomatic action
- Countries that are of the same dynasty
- Countries that have a royal marriage
- Within these brackets, the country with the highest development (taking into account local autonomy) has priority
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 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.
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 the target country's king'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.
Forming a personal union is not technically restricted to kingdoms - Republics (notably noble republics and the Dutch republic) can also become the senior partner (but not the junior partner) in a personal union if all conditions are met. 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.
Personal union through war
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 attain 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 one 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. Ie: Austria can gain this CB on both Bohemia through the "Bohemia must accept Austrian overlordship" mission, and on Hungary through the "The decline of Hungary" mission. The Netherlands can gain this CB on England or Great Britain through the "Invitation to $MONARCH$" event.
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 the player'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).
Claim on Throne or Restoration of Union Casus Belli Through Missions
- Aragon may gain this casus belli on Portugal if they complete the "Crowns of Iberia" mission by controlling most of Iberia. Requires the Golden Century DLC.
- Navarra can reject the aforementioned union with Aragon while under the control of a player, gaining a Force Union casus belli on Aragon instead.
- Navarra has the mission "Claims in Gascony" which allows them to receive the Force Union casus belli over France if the Gascony area is conquered. Requires the Golden Century DLC.
- Castile/Spain can gain a Restoration of Union casus belli on Portugal if it completes the mission "Claims in Aragon" by subjugating Aragon. Requires the Golden Century DLC.
- England and Great Britain have the mission Strategic Control which gives the Force Union casus belli on France. Does not require any additional DLC. They may also declare war with the Restoration of Union CB by event very early in the game.
- Poland can gain the Restoration of Union casus belli on Hungary if it completes the "The Commonwealth" mission.
- Poland can gain the Restoration of Union casus belli on Bohemia if it completes the "Conquer Silesia" mission.
- Lithuania can gain the Restoration of Union casus belli on Muscovy if it completes the "Secure Novgorod" mission.
- Lithuania can gain the Restoration of Union casus belli on Poland if it completes the "Conquer Livonia" mission.
- Spain can gain the Restoration of Union casus belli on Austria if it completes the "Spanish Netherlands" mission.
- Spain can gain the Restoration of Union casus belli on England/Great Britain if it completes the "Spanish Armada" mission.
- Austria can gain the Restoration of Union casus belli on Bohemia if it completes the "Secure Electors" mission.
- Austria can gain the Restoration of Union casus belli on Hungary if it completes the "Decline of Hungary" mission.
- France can gain the Restoration of Union casus belli on Castile/Spain if it completes the "Spanish Succession" mission.
- France can gain the Restoration of Union casus belli on Naples if it completes the "Conquer Milan" mission.
- Hungary can gain the Restoration of Union casus belli on Bohemia if it completes the "Bohemian Borderlands" mission.
- Hungary can gain the Restoration of Union casus belli on Austria if it completes the "Weaken Austria" mission.
- Hungary can gain the Restoration of Union casus belli on Naples if it completes the "Control Bosnia" mission.
- Hungary can gain the Restoration of Union casus belli on Poland if it completes the "Weaken Poland" mission.
- Bohemia can gain the Restoration of Union casus belli on Hungary if it completes the "Conquer Nitra" mission.
- Bohemia can gain the Restoration of Union casus belli on Poland if it completes the "Polish Diplomacy" mission.
- Main article: Subject_nation#Subject_interactions
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 player does not receive any income from lesser partners in a personal union.
Ending a personal union 
A personal union can end in any of the following ways:
- Incorporation of the junior partner into the senior partner, by integration or inheritance
- The junior partner declaring independence and winning the war
- The ruler dying while the junior partner has a negative opinion of the overlord
- Pretender rebels enforcing their demands in the junior partner
- Rebels enforcing demands that change government type
- Peacefully abandoning a PU with a lesser partner at the cost of:
- 25 Prestige
- Truce of 5 years between the former overlord and the released country
- Opinion penalty linearly scaling with liberty desire from -200 to 0 for 0% to 100% liberty desire, respectively.
- The following does not break a personal union
- Religious rebels enforcing demands that change the state religion to a heathen religion (However it is not possible to integrate the junior partner anymore, unless they are also converted to the same religious group)
- Changing religion to a heretic religion through rebels
- Changing government type to a republic/theocracy through Decision
- Ruler dying regardless of over 50% Liberty Desire
- Going Revolutionary Republic while having PU
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:
- Union has been stable for at least 50 years.
- Junior partner's opinion of senior partner is at least 190.
- Liberty desire is less than 50%.
- Senior partner has more provinces than junior partner.
- Junior partner is at peace.
- Junior partner has control of its 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:
- Junior partner Liberty desire is greater than 50%.
- Junior partner does not have control of its 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:
The full formula for diplomatic integration cost is:
The senior partner invests diplomatic power into the integration process each month at the following rate:
- +1 base rate
- +1 if same religion
- +1 if in same culture group
- ±1 per positive/negative point of 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:
- +5 prestige.
- "Annexed Subjects" national modifier for 10 years giving -3 diplomatic reputation.
- −[25 relations with other HRE members, if the junior partner was in the Holy Roman Empire.
- Control of all the junior partner's provinces, army, and navy. This may put the senior partner over its force limit.
- Control of the junior partner's former vassals, colonial nations, and protectorates.
- Cores on all the junior partner's core provinces.
- 60% 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, junior partner 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:
- +1% for each point of Stability
- +5% for each point of Diplomatic reputation
- +5% if in same culture group
- −1% for each province of the junior partner
If 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.
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 and be assigned a new ruler of the same dynasty. 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 player 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 player'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.