Alliances in Crusader Kings 3 were established through arranged marriages.
People rarely married for love in medieval times, and in Crusader Kings 3 you'll have to negotiate a fair number of unions if you want to boost your military might. Family members are your ticket to the top, but there are a few factors you'll need to consider if you want other countries to accept your arrangements.
To begin with, alliances are made by marrying direct members of your family and their children. Nephews, nieces, cousins and other family members can sometimes be used, but they must be higher in the line of succession. Your children and grandchildren exceed the most in the pecking order and make the best deals.
Keep in mind that not all nations will be interested depending on your position in the world of Crusader Kings 3. Nations that are more powerful are less likely to accept an arranged marriage, so if you are looking to build an alliance with a ruler who has a massive army, you might want to check your expectations. When you right-click on a character and select “Find Spouse” or “Plan Marriage,” all possible candidates will be listed and you can use the drop-down menu to filter by Alliance Power. Anyone not listed here will not agree to your terms, so if a nation you are observing isn't available in this menu, you won't be making an alliance with them anytime soon.
Don't give up hope just yet; there are a few tips. On the one hand, the further down the line of succession the two candidates are, the more likely the arrangement is to go through. You don't want to be too greedy essentially. Additionally, everyone in the game is trying to preserve and expand their family dynasty, so the lower someone is in the pecking order, the more likely you are to convince someone to accept a matrilineal marriage if you marry one of your relatives. (or the other way around if you are in a matriarchal society: you want to make sure that the children of the union are born in your house as often as possible).
That said, there may be times when you need to build an alliance in Crusader Kings 3 in a hurry. It is up to you to marry your nearest heirs until they are older; they carry the most weight and can lead to the most desirable matches in a pinch. Be aware that the more alliances you have in Crusader Kings 3, the harder it will be to create new ones. Additionally, alliances disappear when a ruler dies, unless one spouse becomes the new ruler. You reestablish alliances often, so keep an eye on your notifications to make sure you're not about to start a war after an alliance disappears.
You can check your and anyone else's alliances from their character panel, listed to the right of their claims with large, bold banners. Your allies will also have flags on their banners when viewed on the map if you need to remember where they are. You can summon your allies into battle by right clicking on their banner (from your character panel or map), or through the stained glass alert icon at the top of the HUD. Summoning alliances will cost you Prestige in Crusader Kings 3 if you are the attacker, but will cost nothing if you are defending.
Also remember this: when you compare the strength of the army to that of another leader, your alliances are taken into account, so if he says your enemy forces are superior, you will have to gather more alliances or expand your territory to generate larger withdrawals. Additionally, allies won't go to war with other nations they're aligned with or vassals, so use this to your advantage if you want to keep some annoying nations at bay.
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