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Alliances are military relationships between two countries promising to aid each other in cases of war. Alliances take up 1 diplomatic relation. Alliances can only exist between countries of the same rank. Great Powers can never have any allies at all (instead they will have to defend themselves and rely on guarantees or subject relationships to protect others).
When your country changes rank through acquiring more cities, all existing alliances will be transformed into guarantees (there will also be a confirmation dialogue if demanding land in peace would result in a rank change).
When an alliance is broken, it will give a casus belli to the the side being left against the country, who breaks it.
Defensive leagues are purely defensive alliances formed by a city state or minor power. The country forming the defensive league is the leader, which can invite any city state or minor power in their defensive league.
Call to arms
In Imperator you are not able to refuse a call to arms from an attacked ally. If you want the aid of another power you need to be ready to join their wars. You are however under no obligation to join the war efforts.
Guarantees are a relationship through which a powerful country can guarantee the independence of a weaker one. Guarantees take up 1 diplomatic relation. 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.