An alliance is a bilateral military agreement between two nations. When a nation creates an alliance with another nation, the nation receives a diplomatic relations bonus with the ally, but it also comes with obligations: a nation's allies are called to arms (CTA) when an alliance member has war declared on them (a defensive call to arms), or when they choose to call their allies into their own war of aggression (an offensive call to arms).
Generally an alliance is a good-faith pledge to come to the aid of the ally during a time of war. There are exceptions when an ally might refuse a call to join an offensive war (such as being in debt or having a high relationship value with the nation the player is planning to attack), or may not be able to respond to a defensive call-to-arms (for instance, if it is already involved in a war on the side of the aggressor nation).
- 1 Forming an alliance
- 2 Call to arms
- 3 Ending an alliance
- 4 Other relations effects of alliances
- 5 Invite to Federation
- 6 Support independence
- 7 包围网
Forming an alliance
The player may ask a nation to form an alliance through their diplomacy screen. As long as there are more positive reasons than negative reasons, the nation will accept the military alliance. Being in an alliance gives a major relationship boost (+50 Relations ) between the two allied nations.
Certain attitudes of the other nation (such as hostile) may preclude them from choosing to be the player's ally. Setting an allied nation as a rival will automatically break the alliance with this nation and prevents the player from allying again. Additionally, if either country has accepted a peace deal annulling treaties with the other, they cannot enter an alliance together for 10 years after the peace deal.
Maintaining an alliance uses a diplomatic relations slot for each nation. If a nation has more diplomatic relations than its diplomatic cap (for most nations this is initially 4), maintaining agreements costs 1 diplomatic power a month for each excess nation.
Alliance with AI
AI nations will only accept alliance offers if they have more positive than negative reasons for doing so:
†: Distance between borders can only be seen in debug mode. It is possible to estimate distances using the fact that diplomats travel at a speed of 20 distance per day between capitals.
Call to arms
The current war leaders in a war may send a call to arms to their allies at any point in time, asking them to join all their current wars in which they are the war leader. Any co-belligerent can also call his allies to war. If no decision is made in regards to a call to arms, it will automatically be declined in two months. The attacker may only call in allies during the first 30 months of the war.
If war is declared on a nation, it will automatically call on all its allies to come to its defence. Allies who answer a defensive call to arms will not suffer any penalties for breaking a truce, attacking nations they are in a royal marriage with, or have good relations with. Nations who refuse to join will get -25 prestige and lose the alliance.
When a nation declares war, it has the option of individually calling its allies into the fight. Allies who answer an offensive call to arms will suffer all penalties for breaking a truce, attacking nations they are in a royal marriage with, or having good relations with. The aggressor has the option of declaring each of the enemy's allies as co-belligerent or not; if not, taking that country's provinces costs double warscore and diplomatic power as well as additional aggressive expansion penalty, but they cannot call their own allies.
Players that are fielding any call to arms from an AI nation must be very careful about these penalties. As the player will enter all wars that the AI nation is leading, this may incur severe penalties on the player, though the same mechanic could be used against the AI nations as well. Defensive calls to arms are the most common culprit here because they are sent out automatically. For an offensive call to arms, AI nations will only consider the player's ability to join one specific war, and will not evaluate any secondary effects that may result from the player joining all the wars on the side of the AI.
Answering the call to arms brings a nation into the war on the side of its ally as a junior partner in the conflict. Because the ally is not a war leader, it can only negotiate peace with the opposing war leader and only on behalf of itself and its vassals. This is called a separate peace and choosing to make a separate peace will give -30 Relations penalty with co-belligerents, as well as -10 trust. Warscore for separate peace is calculated solely from occupations and blockades and will not consider battles or ticking warscore from wargoals.
Declining a call to arms will result in -25 Prestige and the cancellation of the alliance, as well as a -50 Relations penalty for refusing to join their war, plus -50 Relations for dishonoring the alliance. Refusing a call to arms from a nation that the nation is guaranteeing will result in an additional -25 Prestige penalty (for a total effect of -50 Prestige if the nation also had an alliance). In addition, declining a call to arms will give you -1 diplomatic reputation for 10 years.
Conflicting call to arms
In the war declarations window, it's possible to see what will happen if there are conflicting calls to arms. Usually, the nation will only consider joining the defensive call to arms and not the attackers.
Avoiding call to arms
A nation cannot be called at all if accepting would put it in a war against any nation it is already in a war with, either as an ally or an enemy.
AI nations will only send the player a call to arms if another AI in the position of the player would have accepted that call. A combination of the following options can be used to prevent the AI from sending an offensive call to arms:
- Have a truce with the AI's war target
- Have a royal marriage with the AI's war target
- Make the AI's war target have a high opinion of the player. This is typically the easiest to achieve of all options.
- Have low trust of the AI
- Be in another war
- Be far from the war target
- Be in debt
- Have low manpower
- Have occupied or besieged provinces
- Have high war exhaustion
In previous versions, it was possible to "cheat" by declaring other wars so the player could not answer calls to war. That has been stopped and the player must accept calls to arms or take the malus for declining.
Calling the AI into wars
Ask an ally to join a war. This action does not require a diplomat. An ally called into a war cannot call in his allies, unless it is a defensive war and the ally was named a co-belligerent by the attacker (note that the Emperor defending the empire is always considered a co-belligerent and will call in its allies. Coalition members are also always considered co-belligerents, but they cannot call in their own allies). If an AI will join it or not is shown in the declare war window.
The factors that influence an AI nation's proclivity for accepting a call to arms are
†: Actual monthly income may differ from amount reported in ledger
Among the factors that influence the AI's behaviour above, nation is in debt is generally the easiest one for human players to manipulate. If an AI ally is reluctant to join a war due to debt, establishing a maximum subsidy can significantly reduce this negative factor once the ledger registers the increased income. This allows one to quickly drag the AI ally into war, and the subsidy can be cancelled immediately if there is no concern over the ally's war enthusiasm. Sending a gift before establishing the subsidy may be helpful as well, though there is no guarantee that the AI will use the gift to pay down its debt.
Ending an alliance
Dissolves an alliance with the target country if one exists. After an alliance is dissolved a 5-year truce is established between the former allies. Dissolving an alliance during peacetime will lower relations by -50 with said country. Breaking an alliance during wartime will incur an additional -2 stability hit on top of the relation penalty.
Other relations effects of alliances
Having an alliance with a nation will lower the impact of aggressive expansion. Enemies or rivals may become outraged by a nation's actions, yet its allies will have a less severe reaction. If the nation continues to pile on aggressive expansion, even the closest friend might eventually break their alliance with a nation they consider a hostile warmonger.
Having an alliance can also affect relations with nations other than the ally. Allying with a country that other countries consider a rival will cause a -50 Relations relationship penalty with all countries that consider the ally their rival.
Being allied with a nation slowly builds up trust, which is factored in most diplomatic actions. It is especially important to the AI when weighing whether or not to accept an offensive call to arms.
- 主条目：Subject nation
Vassals and personal unions experience a special non-negotiable alliance with their suzerain for the entire duration of their vassalage/union. A war on one necessarily involves the other, unless the overlord has enabled scutage.
Colonial nations can fight wars without their overlord becoming involved, so long as the war is with another colonial nation or independent Native American nation. The overlord can enforce peace to join any such war, which will also bring in the overlord of an opposing colonial nation. Colonial nations attacked by any other state automatically call their overlord in, just like vassals.
Two subjects of the same overlord may enter into a special alliance with each other. If one nation declares an independence war on its overlord, the other is called in. On success, both become independent.
Vassals, personal unions, and colonial nations can also get support for independence from independent nations. This acts as an alliance in case a subject declares an independence war against its overlord. In cases where multiple subjects are allied with one another, all nations supporting the independence of any of the allied subjects will be dragged into the independence war against the overlord.
Protectorates are not called into their overlord's wars, but their overlord will be called to defend them if they are attacked.
If Japan has the Shogunate government type, its vassals with the Daimyo government type can declare war on each other with the Sengoku casus belli without involving Japan in the war. See the Japan page for more information.
Invite to Federation
Federations are multilateral defensive alliances available only to natives with the native council government. If a federation member is attacked, all members are called in. Federations are not mutually exclusive with regular alliances. The leader of a federation gets a bonus to diplomatic reputation and morale.
Join the independence war against the target country's side if this country as a subject declares war against its overlord. The target must accept the offer of independence and is unlikely to do so if it likes its overlord. If war breaks out, the support is converted to a regular alliance.
- 其他国家拥有对目标国家至少-50的“侵略扩张”（AE）负面关系时，对目标国家的态度会转变为 “愤怒”，但 宿敌则不会转变。
- Coalitions form easily as a result of aggressive expansion. Players should avoid expanding too much into the same area to reduce the likelihood of having a huge coalition forming against them.
- Countries cannot join a coalition against a country they have a truce with, and a coalition is dismantled after a coalition war. Players can use this to their advantage by attacking a coalition while it is still small, dismantling it and preventing the members from joining the coalition again for a while. To reduce further coalitions against the player, they should only make demands that don't increase aggressive expansion, such as money, releasing of nations, returning of cores and humiliation.
- The player may use threaten war against a coalition member to get a truce timer with another nation, this will make the nation leave the coalition. This can be used to break down coalitions.
- The player should prefer to attack a coalition rather than let it strike first since the coalition CB gives a -30 modifier for the coalition to keep fighting, thus making the war longer and more costly.
- If a coalition can reform after a truce, the player should prepare and be ready to attack before the coalition has a chance to reform after the truce ends and they can enter the coalition again.
- Diplomacy can be a useful tool to dismantle coalitions. Any nation with a modifier of -90 or better can potentially be brought up to +50 with a combination of improving relations, gifts, and subsidies, which will cause them to leave the coalition. Doing this to a very large and powerful coalition member may cause the rest of the coalition to collapse outright, as none of them want to be the target of your wrath without a powerful neighbour to back them up.
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