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Nations can sign various treaties with one another to agree to help one another in aggressive and defensive wars.
Alliances are military agreements between two countries promising to aid each other in case of war. 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.
When an alliance is broken, the alliance breaker will get truce 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.
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. They can also remove members. All members automatically join wars when any member is 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.
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 gives the corresponding penalties.
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.