The alliance system in EU3 is bilateral, which means that all alliances are treaties between two countries. This means that it is perfectly possible for a country A to be allied to countries B and C that are bitter enemies. Country A might also come in a situation where they have to choose which side to support in a war. In this case, whichever side is turned down (possibly both) will automatically have its alliance ended, and a state of war will exist.
When someone declares war on a country in an alliance, the victim's allies can either declare war on the aggressor or end the alliance (without spending a diplomat). If an ally declares war on another country, you have the option to join the war on their side (without spending a diplomat).
Likelihood of acceptance
If you seek an alliance with another country and it says "impossible", there are several reasons this could be the case.
- They already have 3 allies. AI players are limited to 3 non vassal alliances, the player can have an unlimited number
- Your Relations with the other nation are -100 or worse
- Your Reputation affects your chances to make diplomatic moves
- Your Prestige score affects your ability to attract allies
- Your trust is too low
- They have a different religion to you (before 1650)
- they have ongoing wars they don't want to drag you, or get dragged themselves, into (DW only)
- You are a Rival, or a Threat to them
- They lost an earlier war to you, or have cores on your land, or they plan to attack you (e.g. via missions)
- They might be "too far" away
- The diplomacy rating of your ruler
- If trying to ally with a Vassal, there must be a route from your capital to theirs. The route can cross the sea, but not other countries (even your other vassals) and detached provinces don't count - it has to be capital to capital. For overseas vassals, you must have a connection to a coastal province so you can connect with overseas vassals.
Cascading alliances are an interesting feature of Divine Wind in which the Alliance Leader in a war is capable of summoning its allies to assist in conflict, even if it is not the nation which originally declared war or had war declared upon. This can result in small wars becoming extremely large as the webs of alliances come into open conflict.