Crusader Kings 3 is not a fair game. Whether players choose to start in 867 or 1066, some nations are simply bigger and meaner than others. Players who choose to start as a small-scale ruler at the edge of an empire will have a tough time staying independent, assuming they don't take advantage of alliances.
An alliance can be a useful way to level the playing field between big empires and petty kingdoms. By banding together, several duchy-level rulers can field an army the same size as a neighboring empire. However, rulers don't hand out alliances for free; consequently, players must understand how the alliance system in Crusader Kings 3 works to take advantage of them.
Bonds of Blood and Marriage
To put it simply, other rulers in Crusader Kings 3 will only consider an alliance with another ruler if they're a close relative. This can be a direct relative like a sibling, child, or parent; a slightly distant relative like a cousin, nephew, or uncle; or an in-law relative connected by a marriage between close relatives.
In fact, a marriage between the close relatives of rulers automatically creates an alliance between the two rulers, and the game takes this fact into consideration when it decides whether an NPC ruler will accept a player ruler's marriage proposal. And since alliances go both ways, players should be equally careful about choosing good matches for their kin.
When it comes to alliance making, betrothals are just as good as an actual marriage, so players can promise their young children to foreign rulers to get the assistance of their armies. This also works if the person being married isn't technically a part of the ruler's dynasty or house, like a bastard or the child of someone who marries into another ruler's house.
In fact, such individuals are more useful for marriage alliances since they'll never inherit the ruler's titles. Players can tell which close relatives aren't in their ruler's house because the blood droplet that symbolizes close kin is red instead of green. This can also be a way to get rid of relatives with bad attributes while still making use of them.
If two rulers are closely related without a marriage involved, they don't automatically ally with each other. Players must choose to negotiate an alliance with them, and NPC rulers will accept or refuse based on their Opinion, relative strength, how many other alliances players have, and so on.
The number of a ruler's current allies can be a big factor in the game's decision-making. Only war leaders can call allies into their war, but the danger is that a ruler with many allies may be called into many wars all at once.
The one exception to the family and marriage alliance rule is a Diplomacy Lifestyle perk called Defensive Negotiations. Once a ruler picks this perk, they can propose a single alliance with another ruler without having to marry anyone off. However, rulers can't propose an alliance in this way when they're already at war, while marriage alliances will still take effect during a war. Note that the game calls this interaction "Propose Alliance," while "Negotiate Alliance" is for relatives.
How to Invoke an Alliance
There are two main ways to make use of alliances: internally and externally.
The external alliance does what most players expect. If a ruler goes to war in Crusader Kings 3, they can call their allies to join it. The ally can then choose to accept the call to war or reject it, but in general NPC rulers will accept. NPC rulers are especially likely to accept if their ally is in a defensive war, and they're especially likely to reject if two of their allies are at war with each other.
Calling an ally to a defensive war has no cost involved, but calling an ally to an offensive war comes with a Prestige cost. The amount of Prestige required is based on the ally's rank. The ally then has an obligation to contribute in some way to the War Score. If they don't, the war leader can demand that they provide Gold, Prestige, or contribute to the War Score within 1 year. If the ally ruler still doesn't do anything, they'll get a big -50 Opinion with the war leader and lose a level of Fame.
An internal alliance works differently. This is where a liege allies with one of their direct vassals. Lieges can't call vassal allies to war, and vassals can't call their lieges to war. Instead, a vassal ally can never join a faction against their liege and a liege ally can never revoke a vassal's titles without a valid (non-Tyrannical) reason.
To break a marriage alliance, players must break the marriage, either by ending a betrothal or by forcing married characters to divorce. This can be easy, hard, or impossible depending on how the player ruler's Faith views divorce. However, if their Faith prohibits divorces, a player ruler can always make sure the "Til death do us part" clause comes up sooner rather than later.
Players can also break an alliance by going to war with their ally, but doing so is a bad idea. Declaring war on an ally costs 250 extra Prestige, 1 level of Fame, and adds a -25 Opinion penalty to every character in the game for 3 years. Finally, alliances break automatically when a ruler dies, but players can sometimes renew them if the connection to the new ruler still involves close family members.
One other way rulers can call in allies is through dynastic connections. If a ruler is a Dynasty Head, they can call in any Dynasty member outside their realm no matter how distantly connected they are. However, calling in Dynasty members costs Renown instead of Prestige, the cost is the same whether the war is offensive or defensive, and the amount of Renown needed depends on how many houses the Dynasty has. On the other hand, a House Head can call other House members to war, and this interaction carries no cost.
Crusader Kings 3 is available now on PC, PS5, and Xbox Series X/S.