A vassal is a country which has assumed a subordinate relationship to a dominant country, called the "suzerain". Historically, being a vassal almost always implied providing military assistance to the suzerain when called, and sometimes the payment of tribute.
In EU2, vassalization is a kind of diplomatic agreement in which the obligations of military assistance and tribute are reversed from history. A vassal always pays tribute from its province taxes, but is not necessarily a part of the suzerain's alliance.
Effects of Vassaldom
Any time a vassalization exists, it has effects on the vassal and on the suzerain. In addition, there are a few effects on suzerains that obtain not based on any specific vassal.
Effects on Vassals
A country which is a vassal has the following limitations:
- It cannot enter any war on the suzerain without breaking its vassaldom.
- It gives the suzerain one half of its province taxes.
- It cannot join any alliance except for the one which its suzerain is in. Note that it can create a new alliance, and it can also be in alliances not containing the suzerain, if the suzerain dishonors the alliance and it does not.
- It cannot make any separate peace treaty in any war which its suzerain is also in. This includes force annexation.
Effects on Suzerains of Specific Vassals
The suzerain has these benefits and effects:
- It cannot enter any war on the vassal without breaking the vassaldom.
- It gains one half of the vassal's province taxes.
- It may gain a fraction of the vassal's manpower.
The manpower effect varies with the suzerain's Centralization domestic policy: at max centralization, a country gets no bonus techspeed or manpower from any vassal. At max decentralization, it gets 50% of the vassal´s manpower. Also, the vassal must be on the same continent: vassals on other continents do not give any manpower bonus regardless of centralization.
The following effects happen to any country that is a suzerain of one or more vassals:
- It cannot be diplomatically vassalized.
- It may gain a tiny bonus to techspeed.
- It may gain a stability cost reduction up to 20%, depending on its centralization domestic policy, and the number of vassals it has on the its continent. See the article on stability cost for details.
- It gets a manpower multiplier on its own manpower.
Due to a bug in EU2 1.09, when a country has at least one vassal (regardless of where the vassal is), the country gains the bonus to the manpower based on its own manpower, as if it were its own vassal. (This is in addition to whatever it gets from vassal(s).) At full decentralization, this gives it in effect a huge +50% manpower bonus.
Gaining a Vassal
There are several ways in which a country can gain a vassal. These are reviewed in this section.
Releasing a Vassal
When a country owns a province which is a "minimum" province of a country defined in revolt.txt, it may be allowed to release the new country as a vassal. A country can release a vassal if these conditions are true:
- it owns and controls at least one "minimum" province of the vassal
- the vassal does not currently exist
- the other conditions encoded in revolt.txt hold: the dates allow the vassal to exist, no other countries exist which disallow it, etc.
When a country releases a vassal, all provinces which it owns and controls which are "minimum" provinces of the vassal are granted to the vassal. (If your country's capital province is granted, your capital will be moved to the lowest numbered province that you still own. If you grant all of your provinces to a vassal, you lose.) The vassal starts out with +200 relations to the suzerain; other aspects of the new country are done according to the normal process of new country creation. The new vassal does not start out allied with you. If you want a newly released vassal in your alliance, you'll have to spend a diplomat to do that.
AI countries never release vassals voluntarily.
There are events which force a country to release a vassal. (The command is called "independence".) There are no random events which do this, and few scripted events, but a few countries have them. For example, the Burgundy inheritance event begins with the freeing of Gelre, Friesen, Kleves, Hanover, Cologne, Lorraine, and the Palinat, if Burgundy owns the appropriate provinces.
Forced vassalization (abbreviated FV) is possible in certain conditions when a country offers peace:
- It must control the enemy capital
- It must have at least 70% in warscore against the enemy.
- The enemy must not be someone else's vassal.
If the enemy country accepts a peace proposal with a force vassalization, it becomes the vassal of the country which made the peace (which had to be the one controlling its capital). In addition, it breaks any alliance it is in, and all royal marriages. Relations between the two countries will also rise.
AIs will demand vassalization in war if they have the warscore for it, but this is somewhat unusual since they'll normally make peace long before they get 80+ warscore. Usually AIs only end up vassalizing 2-province countries.
Diplomatic vassalization (abbreviated DV), or "diplovassalization" as players sometimes call it, is a diplomatic action. A country can attempt to diplomatically vassalize a second country only when all of the following conditions hold:
- The countries are members of the same alliance.
- The countries have a royal marriage.
- Relations between the countries are at +190 or higher.
- The target country is not currently at war.
- The target country is not a suzerain (in this case the "offer vassalization" menu will be active, but the attempt will automatically fail).
Even when all conditions above hold, diplovassalization does not succeed automatically. Chances are (or seem to be) increased for the would-be suzerain by the following conditions:
- Having a superior monarch diplomatically.
- Having a superior economy (this seems to be done by monthly income, so, try to vassalize in a month in which you doing a lot of looting).
- Having a superior military (morale seems to matter, so put your maintenance slider to 100% in the month you try to vassalize).
- High stability.
- Low badboy value.
If a diplovassalization fails, relations will decline by 30 points. There is no other bad effect, so if you've got a great diplomatic monarch and money, you can keep sending gifts and trying until you succeed.
There are several ways in which a country can cease to be a vassal. They are:
- If a suzerain gets annexed. Any vassals it has at the time are freed.
- If a suzerain and its vassal enter the state of war against each other.
- Either party may choose the "Cancel Vassalization" diplomatic action, so long as its stability is at least 0. AIs will do this seemingly based only on low relations, not size. Thus, for example, if Nippon can vassalize China, it can maintain China as a vassal throughout the game. A force vassalized country cannot cancel the vassalization during the consequent truce.
- If the suzerain attempts diplomatic annexation of the vassal, and the attempt is a failure, there is a chance that the vassal will break vassaldom.
The country that cancels the vassalization will get -50 relations to countries it shares state religion with, in addition to -3 stability.