Alliances are military agreements between two countries promising to aid each other in case of war, both offensive and defensive. Allies get +50 opinion of each other while the treaty exists and will always join in a defensive war, but may turn down an offensive call to arms if they have a high opinion of the target, do not have military access, or are otherwise not prepared to go to war. It is not possible to directly declare war on an ally, but it is possible to go to war with an ally by declaring war on one of their other allies or subjects, which will immediately break the alliance. Alliances can exist between countries of different ranks, with the exception of Great Powers. Great Powers may never form or maintain an alliance; instead they must defend themselves and rely on guarantees or subject relationships to protect others. If a country becomes a Great Power, all pre-existing alliances are automatically turned into guarantees.
An alliance can be broken at any time as long as the two countries are not in a war together. When an alliance is broken, the alliance breaker will get a truce with the former ally, but not the other way around; instead, the former ally will get a casus belli against the country who broke the agreement, with no truce to prevent its immediate use. It is possible to circumvent this truce and declare war on an ally by instead declaring war on one of their subjects, which will immediately break the alliance with no penalty. AI-controlled countries will break alliances if relations become too poor and/or they if they covet some of their ally's territories; there will always be some warning that this is happening to either try to convince them to stay in the alliance or make the appropriate preparations.
Alliances take up 1 diplomatic relation. There is no hard cap to the number of alliances that a country can have, though AI-controlled countries will be increasingly hesitant to create alliances with a country that already has a lot of allies and is over the diplomatic relations cap.
Strategies on breaking enemy's alliances
- Remember that subject nations cannot have alliances. This becomes more crucial when you have an ally which is also allied to your target, as said ally will attack you when you declare war on the target.
- Normally, only the main war target can call in their allies. If there is a country with an ally that is too strong to take on, try looking through their other allies to see if they can be pulled in as a secondary participant in an easier war.
Defensive leagues are purely defensive agreements formed by city states or local powers. The nation forming the defensive league is the leader, and can invite other city states or local powers to join (or alternatively accept their requests to join). All members get +25 opinion of each other and automatically join wars when any member is directly attacked, unless the attacker is also a member of the league. A country can only be part of one defensive league at a time, but being in a defensive league does not preclude participating in other treaties such as alliances or guarantees, including with other members of the defensive league. Like with alliances, it is not possible to directly declare war on another member of a defensive league, but it is possible to go to war with another league member by declaring war on on of their other allies or subjects. Doing so will not remove either party from the defensive league.
A country can leave a defensive league at any time with no penalties, and potentially attack another member of its former league immediately afterwards.
Strategies on breaking up enemy's league
- Remember that only city states or local powers can form leagues. If your target is almost the size of a Regional Power, gift it some of your own territories; you can take them back once you successfully isolate the target and annex it in the resulting war.
- If territorial gifts are not an option, see if the target's fellow league members can be made your subjects.
- As with alliances, a defensive league can only be called in if a member of the league is directly attacked. Try looking through the target's allies to see if they can be pulled in as a secondary participant in an easier war that does not involve their defensive league.
Guarantees are treaties through which a powerful country agrees to protect a weaker one against any external aggression. If a guaranteed nation is attacked, a call to arms will be issued to the guaranteeing nations. War leadership will transfer from a guaranteed country when its strongest (most populous) protector is called; once war leadership has transferred, the new warleader can call in their subjects and allies as well. If a country has a truce with the protecting power, attacking a country that it guarantees counts as breaking the truce and costs 10 stability and 5 aggressive expansion, a smaller penalty than breaking the truce directly. The guarantor cannot directly declare war on the guaranteed nation, but the guaranteed nation can declare war on their guarantor. In addition, the guaranteed nation gets +10 opinion of their guarantor, and the guarantor gets +5 of the country is being guaranteed. A guarantee can only be made by regional powers, major powers, and great powers, and only on countries with a lower rank.
A guarantee can be revoked by the guarantor at any time, which creates a 6 years truce between both countries. If the two countries end up at war, the guarantee will be immediately broken with no truce added.
Call to arms
In Imperator, a player-controlled nation is not able to refuse a defensive call to arms from an attacked ally - if the aid of another power is desired, the country needs to be ready to join their wars as well. However, there is no obligation to mobilize troops or actively participate in the war. An offensive arms can be refused at the cost of breaking the alliance.