Alliances are military relationships between two countries promising to aid each other in cases of war. Alliances can only exist between countries of the same rank. Great Powers may never form or maintain an alliance; instead they must 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 agreement 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.
Alliances take up 1 diplomatic relation.
Defensive leagues are a purely defensive agreement formed by a city state or a local power. The nation forming the defensive league is the leader, and can invite other city states or minor 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.
Guarantees are a relationship through which a powerful country guarantees the independence of a weaker one. 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.
Guarantees take up 1 diplomatic relation.
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 mobilize troops or actively participate in the war.