The fundamental mechanics of diplomacy involve: relations, infamy and interests.
Relations is a value on a scale between -100 and +100 that determines the overall diplomatic standing between two countries. Relations in Victoria 3 are bilateral, meaning that these two countries will always have the same relations score towards each other. This makes it clear where two countries stand with each other, and at the same time applies the mechanical effects on both the AI and the player (even in multiplayer).
The relation number will translate into a relations level, and the different relations levels are as follows (from highest to lowest):
All of these have an impact on the AI’s decision-making in terms of which diplomatic proposals it will accept, which side it will want to join in diplomatic plays, and so on, but besides that there are also limitations on what actions you can take against another country based on your mutual relations.
For example, a relations level of Cordial or above acts as a non-aggression pact: It isn’t possible to start most diplomatic plays against a country with which you have that relation level without first acting to reduce said relations. On the flip side, signing and maintaining a Customs Union with a country requires you to be at or above Cordial relations, and there are other actions that cannot be taken unless relations are at other certain negative or positive thresholds.
Changing Relation Levels
The primary way to change relations is through the Improve relations and Damage relations ongoing diplomatic actions, but there’s many other ways in which relations can be increased or decreased, including various events, diplomatic incidents and the Expel Diplomats diplomatic action, which is a way in which one country can act to prevent another from cozying up to them relations-wise, though at the cost of gaining infamy.
A country has an infamy value that starts at 0, but which may increase to any high number as there’s no upper cap on it. As a country’s infamy increases, other countries will become more wary, resulting in various diplomatic penalties for the infamous country. If infamy exceeds the pariah threshold (which is currently set to 100) the country becomes a potential target for a special “contain threat” diplomatic play where the great powers step in to “restore order”. Infamy decays slowly over time, and its rate of decay can be increased if the country has a large amount of unallocated Influence capacity, representing that capacity being put to use trying to salvage the country’s global reputation instead.
Infamy in itself should be understood as a measure of how concerned the great powers are about a country, and as such, country rank has an effect on how much infamy a country gets when it commits a diplomatic transgression against another. Generally speaking, the lower the rank of the two countries involved, the less infamy will be generated, as the great powers care a lot more about actions taken by and against other great powers than they do over two minor powers being engaged in a local squabble.
Ultimately, what this means is that infamy doesn’t just have a global effect, and where you’re accruing it matters. If you keep taking actions that destabilize a particular strategic region, you can expect to quickly become very unpopular with both the locals and any outside powers that have taken an interest in it.
Infamy is tied to the diplomatic incident mechanic. In the vast majority of cases, any action a country takes (for example demanding land in a diplomatic play or violating a neutral country’s sovereignty during war) that increases infamy will also create a diplomatic incident localized at a particular strategic region on the map.
For example, starting a diplomatic play to demand a colony in West Africa will result in a diplomatic incident occurring there. Whenever a diplomatic incident happens, the country that caused it immediately suffers a penalty to their relations with all countries that have an interest in the region, with the amount of relations lost based on the amount of infamy attached to the Incident in question.
Interests is a mechanic that determines whether or not a country has a stake in a particular strategic region and plays into numerous different mechanics such as diplomatic plays, colonization and the aforementioned diplomatic incidents.
Interests do not provide any inherent benefit to a country besides the ability to throw their weight around in a strategic region, and can actually be a bit of a double-edged sword in that a country with interests all over the world may get dragged into a lot of local conflicts. Ultimately, interests are intended to simulate such historical occurrences as why certain parts of the world simply got a lot more attention from the great powers than others at particular points during the century that Victoria 3 covers, and to make nations act and care about things in a way that makes sense according to their national self-interest.
A country can gain an interest in a region in one of two ways: either automatically by having a geographical presence there (owning land or controlling subject nations in the region) or by using a declared interest.
A declared interest is a country quite simply saying that, regardless of their lack of a geographic presence, a strategic region is still of importance to them, perhaps because they plan to colonize it, or because they want to prevent a hated rival from expanding into it. A country can declare an interest in any region that is either adjacent to a region where they already have an interest, or which they can reach through the support of their naval supply network. The number of declared interests that is available to a country depends on their rank – a great power can choose to have its fingers in a great many pies, while an insignificant power is limited to acting only in regions where they already have land.