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This is a guide to various aspects of the game, which can come under the umbrella term 'Diplomacy'.
- There is also War and Peace: Secret Master's Guide to Diplomacy and Imperialism, a guide for specifically using influence to prevent great powers intervening in your wars.
Who should I ally?
As a general rule, the stronger an ally you can get, the better. There is however some exemptions. In general, the AI is very bad at transporting troops across bodies of water. Therefore, it is better to ally someone, who has direct land access to you. E.g. 
Austria might be a better ally than 
United Kingdom for a central European country, even though Britain technically is stronger.
It is also better to choose someone, who does not have a lot of natural enemies. 
Ottoman Empire might be the strongest ally you can get, but will be attacked so much, that it is just not worth it to risk the prestige hit by not honoring a call to arms.
Last but not least it is better to choose someone, who naturally has the same enemy as you. E.g. allying 
France as 
Russia if you intend on attacking 
Prussia. When considering if they want to join a war or not, the AI have a 'political considerations' modifier which will be positive, if they also have an interest in attacking that specific nation.
For small nations, there is however often not a choice in who should be your great power ally. Often you will just get sphered by a nation, that will turn out to be your main protection.
Who can help who in wars?
If you are attacking another nation, there are 3 scenarios where another nation can join the war on the opposing side:
1) They are allied to nation you are attacking. That is stated clearly in the 'declare war' window.
2) A great power has at least a 'friendly status' and chooses to intervene in the war. This is not clear when looking at the 'declare war' window.
3) A much larger nation becomes war leader after joining for either reason one or two, and they call in their allies.
If you have checked the level of influence of all 8 great powers, the allies of all the great powers that can possibly intervene, and the allies of nation you are attacking, you should know exactly who you are going to war with.
Influence and Spheres
- general article: Influence
Think of a "sphere" as a group of client-nations that are dependent on the sphere leader for guidance. This is distinct from a "Satellite". A good example from this time period would be the Slavic states of the Balkans, which generally fell into Russia's "sphere". In Victoria 2, members of a sphere form a common market, on which goods are purchased and sold first, before going to the world market. This can be beneficial to ensure resource availability, as well as a captive market for finished goods. It is important to note that tariffs will not apply to goods purchased on this "common market".
How does sphering work?
Great Powers may influence other countries to join their Sphere, which makes forming alliances much easier. 50 points is needed to increase Opinion up until "friendly". 100 additional influence is needed to add the country to the Great Power's Sphere when said nation has a friendly status. 50 influence can also be used to decrease the relation of another great power. If another great power has sphered a nation you will need 100 points and friendly status to remove the nation from sphere. After a nation is removed from the sphere, the Great power will have a friendly status.
A great power status can vary from "hatred", "Opposed", "Neutral", "Cordial", "Friendly" and "Sphere Of Influence". Most nations will have a status of "neutral" with most great powers.
Last but not least, 25 influence can be used to discredit another Great Power, which will make them gain less influence over a certain time period. Influence can only be used to harm other great powers influence, if they are at the same status level. The exception is that it requires a friendly status, to remove a country from a sphere of influence.
Tricks and Tips
Faking Interest
Great Powers may choose to deplete the existing influence of their rivals by establishing a tiny bit of influence at each country where any Great Power has at least 25 influence. Most of the time, the great powers will think that you have an interest in that country and therefore spend their 25 influence by "discrediting" the Great Power trying to advance its own influence. This can result in the destruction of several hundred influence at the cost of 3–5 days of influence. The Great Power may then allot its influence freely in any country where it is not discredited, assured in the knowledge that its rivals are months behind in sphering their own.
Controlling Capitals
If a revolution takes place in one of your spherelings, you will lose all influence. Therefore, it is important to prevent that from happening. If you have a large military (which is most likely if you are a great power), you can place a stack of troops on your spherelings capital during peacetime. That way, rebellions are never going to rise up and quickly seize control of the capital, causing a revolution, before you can react. Your troops might as well be stationed there, while they are not doing anything else during peacetime.