I'm not sure if the inability for a country to declare war under regency council make any sense at all, as I was under an impression that the regency council would normally have full authority to exercise all royal prerogatives unless the will or an Act of Parliament explicitly restricted it. There are known instances where powers of the regency have been legislatively restricted, best example being the case of the British Prince Regent ruling on behalf of incapacitated George III during his late reign. However, that restrictions was limited to a duration of one year only and I believe the power to declare war (which formally remained royal prerogative to this day but nowadays the government will seek public approval before going to a war, though formal declaration appeared to have not been used since the Second World War) was not explicitly restricted.
Rather than completely disable it for Regency Council, I suggest putting the estates or Parliament (latter via the executive Cabinet, who are always composed of MPs and Lords of Parliament in Constitutional Monarchy; this is particularly true in Britain) in active role with respect to declaring the war. Whether they would support the declaration of war would depend on factors. The factors may be the following:
- Value of provinces of the country proposed for declaration of war against to the estates (e.g. potential for wealth as in Development/Trade for Burghers, potential for glories and opportunities (military career, more land) for Nobility, possible new provinces to convert for the Clergy)
- Relative strength of military to the proposed country
- Estate loyalty; negative estate loyalty make it much more difficult to convince the estate to support the war
- Relative level of disorder embodied by rebellions, stability level, et cetera
- Possible disruption to the trade as a result of the war which may or may not make Burghers estate nervous.
- Whether the proposed target is a rival, is Great Power (more especially relevant if the attacker is not a Great Power), have excellent/poor relation, supported rebellion of the other estate (Nobility might take a very narrow view of a foreign power supporting the peasant rebellion in their own lands)
- Even more intriguing, the bribery by a foreign country through the Spy Actions that make them less willing to support the war against that country (and possibly that country's allies as well); conversely they may be bribed into supporting declaration of war against the country that the bribing country is a rival or for some reason. The bribery action would have to be ongoing and active. To make it a bit more interesting, the bribed estate may even agitate for a declaration of war against such country via events with some penalty for failure to propose a DOW before the timer expired (not unlike that event where the Burgher estate request that you declare war on a specific country or something like that). Such event may also occur independently of bribery.
- Whether the target is overseas or not; the Nobility estate might be less likely to support as they tend to be far less involved in colonization than the Burghers are.
And I could go on but you get the idea. (Note 1)
In this scenario, you would see beforehand their leanings (similar to how you can see where the AI enemy countries stand when offering peace to them) with respect of proposed declaration of war before them. In fact, some of the factors used in determining a country's willingness to join the call to war may also be used in Regency Council war declaration mechanic. You would have to use bribes, make offers, and other leverages to win their approval for the war within the Regency Council. There may be some trade-off in negotiating support for the declaration of war prior to the vote within the regency so you would need to carefully weigh the benefit and drawbacks with regards to the members of the Regency Council before making any bribes (immediate effect) or offering concessions (long-term effect; adds ).
The scenario lead to another thing to consider: The Regency Council should be populated, instead of being completely faceless body, with randomly generated personalities similar to Advisors that are drawn from various Estates, with an estate being dominant or even have all seats. In fact, it may even have Advisors and possibly Military Leaders but these personalities need to be also drawn from the estates (similar to how they are drawn from various cultures, different religions, et cetera). Advisors may vote with the estate they belong to unless sufficiently persuaded with offers of concessions (pay raises for Advisor? ;P) or bribes.
The council may have 3 or 5 members; it may even have a range of 3-5 members depending on how big the country are. Obviously that would mean trying to negotiate with the council can get a bit more (financially and politically) expensive in larger country since you have to deal with five instead of three. There could be greater number beyond 5 but I don't know how practical that would be.
Who sits on the Regency Council at the time of monarch's death may be decided beforehand if the player desire (after all, Henry VIII of England named the members in his will before he died and was succeeded by Edward VI who was still a minor at the time of latter's accession to the throne). You would choose from list that contains randomly generated estate personalities, the advisors in employment at the time of selection, and possibly the Military Leaders). These selection may in turn determine the Monarch Power of the regency council. If any member died and were not replaced before your monarch died, the replacement will be randomly generated. It may even be required that at least one member be from Advisors and one member from at least one estate or all estates.
This in turn may also mean that members of the regency may try to take advantage by making some politically controversial demands such as granting more lands to their own estate, at expense of royal power and other estates (which is sure to provoke unhappiness and jealousy, especially if they do not have a seat on the council). I think
All of this, I think, would make for an interesting strategic challenges in preparing for the regency in the event it should occur. I don't remember if monarchs generally lives longer in late-game or their life expectancy is just random but assuming that life spans improves over the time, this would potentially mean regencies would be more likely in early game and thus more frequent. However, whether this will contribute to gameplay value or would create more micromanagement is an open question and that is something that ought to be examined carefully before it may be considered for implementation into the game. Moreover, it is not clear whether this is feasible within the current framework / foundation that Europa Universalis IV is built on, so it may have to wait for the next-generation successor EU5, whenever that will be developed and released.
Regarding the lack of Nobility estate as an issue that we have discussed in context of English Monarchy with respect to the estate regency since their members have been historically drawn from that estate, there is a possible solution. The Nobility estate may be re-enabled as an entity but their interactions and such would be disabled when Parliament is active, with their power being derived from its control of the Parliament instead. Passage of certain issues in the debate may reduce or increase their influence/loyalty relative to the Crown and also could impact the loyalty/influence of other factions. I could go on about that and more but improvements to the Parliament mechanics is another topic so I may just make a new thread about this instead. The point is that, by enabling that estate, it could solve the problem of having no Nobility estate to lead the estate-based regency council.
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Note 1: One interesting thing I just thought about is that If assignment of provinces for estates had been retained, the relative proximity of their provinces to the potential outbreak of war (e.g. border provinces) may make them less willing to support the war, unless that country have relatively high level of Aggressive Expansion. Just a thought, not advocating its return or continued removal.