Alliances don’t seem like a natural fit as a subcategory of Marriage, but in Crusader Kings 2, that is most definitely where they belong. The ONLY way to secure an alliance is to forge a familial bond between yourself and another ruler. This will almost always be achieved through the marriage of your children, but there are always exceptions (like fighting a war to install a new ruler who is already related to you).
From the ‘Select a Spouse’ window (if you have clicked the wedding rings on the character page) you will be shown a blue flag next to any potential spouse whom marrying will create an alliance. Mousing over the flag will tell you who the alliance is with. A black flag will indicate that no alliance will be forthcoming. If you are picking a spouse from the character search window, you will have to look at for any resulting alliance in the diplomacy tab after setting up the proposal there.
It is important to note that not all alliances are created equal. In Crusader Kings 2, alliances are almost strictly for military purposes, and as such, you want to make alliances with rulers who can answer your call to arms. If you forge an alliance with another character who is a vassal, they will not be able to answer your call unless you both share the same liege, so you will want to forge alliances with rulers of independent realms whenever possible (you want an alliance with a ruler who has no liege himself/herself). It is also useful to forge alliances with realms who are in your vicinity. The HRE is a powerful ally, but they may not come trucking all the way over to Ireland to help you conquer the count next door.
You can look at your Ruler’s “Allies” tab to get an idea of which allies may help you should you need it. If the text behind your ally’s name is red, he cannot be called to war, if it is green, he or she can be called, but still may decide not not get involved. Mousing over the ally will give tooltip info that will sometimes indicate why they cannot be called to war. The most common reason is that they are a vassal, and cannot be called to war unless by another vassal of the same liege. Relatives who are your vassals will show up as allies, but you don’t really need to call them to war, as that’s kind of how the whole vassalage thing works.
In the screenshot above, the tooltip says very little, but Caspar Salian cannot be called to war because the only war i am involved in is one that he called me into. This is one place where the tooltips do not always tell you what you would like to know.
Your alliance will last as long as the familial tie between the two rulers lasts. In the screenshot above, Ramon Berenguer’s half-sister is maried to Caspar Salian’s brother. If my half-sister, or his brother, were to meet an untimely end (assuming that there are no other, lesser, ties), the alliance would be off, though any existing Calls to arms would be honored.
Other Marriage Topics