An NPC joined a war and none of his allies are in the war. How was he called into the war? The war in question is a county conquest.
How to determine why someone joined a war
crusader-kings-2
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Here's how it works: If you are their liege, the gains from the war are part of your kingdom. It doesn't matter if they're your king, duke, earl, or even mayor, bishop, or count.
To claim a duchy from somebody else through war (when you don't have a casus belli that lets you claim it directly):
Find out who* has a claim on it by opening the duchy information screen and clicking on "Claimants" to see who has a claim on that kingdom.
Invite one of the male claimants over to your court. (If nobody will come over, or there are no male heirs, you'll have to find another way to get that duchy. Note that if one of your courtiers marries a female claimant that you can't legally give a title to, that female claimant is not your vassal and the newly-conquered duchy will not become part of your kingdom, even if you give your male courtier a title.)
Give that claimant a landed title. Any landed title will do, even a city, church, or castle.
Note that if you land the claimant a city, gaining him a Duchy will found a vassal merchant republic instead of gaining you a new feudal vassal if you are playing version 1.9 of the game or later. Merchant republics are generally considered awesome to have as vassals due to their revenue and tax bonuses, but this might disrupt an existing merchant republic that is already earning you revenue.
Go to war with the ruler of your choice, using the "Claim on duchy X" as your casus belli.
Once you win the war, that claimant will be your duke.
I'm guessing that this will also work with kingdoms if you're an emperor. So far, I've claimed two duchies this way: once by giving away a county to the claimant, and once by giving away a bishopric.
*As revealed in this other answer, you can't actually press the claim in war unless the claim is strong, or the claimant is a pretender (2nd or 3rd in line), or there's already another war going on over the title, or the current holder of the title is a woman (and you have a male claimant) or a child (so there's a regency). You can only use "any claimant with a pulse" if the title is disputed or the ruler is a child; you can only use "any male claimant with a pulse" if the ruler is female; otherwise, you'll need to pick up one of the pretenders or someone with a strong claim: you'll need to do slightly more research to find who they are, and they're often harder to invite to your court.
(Note: if there are three or fewer male claimants, then it's almost certain that they're the heir and pretenders. Just grab one and go on your way. Figuring out the exact claims is more important when there are four or more male claimants.)
Estimate your armies: look at army screen where you raise your own and vassal's levies - look for "unraised" - it is in the middle. Add these 2 numbers and add your current retinue. This will give you the estimation of your forces.
Open Ledger screen (a button with bar diagramms in the left-bottom), go to page 10 - independent states. This can give you an estimation of your opponent's armies. But it may be inaccurate!
Always check for allies using diplomacy view and character view, allies tab.
You may be quite sure to win a war if you have a 3-5 times more potential troops then your opponent.
These factors drive the chances of winning to your side:
- Opponent is involved in other wars. IMPORTANT: these wars should contest other titles, not your target title. Or every member of that war would be hostile to you.
- Opponent is in civil war - there is some rebellion going. You just have to avoid fighting with rebels.
- Opponent's armies are depleted after previous war - quite hard to notice, but you may deduce that fact based on the event on the map
- The contested title is far away from enemy's capitol, ideally - separated by sea. It would be hard to deliver the entire army and you will defend and use terrain bonus, because the warscore ticking would make the opponent to attack you.
- You have a strong fleet and outnumber the opponent at least 2:1 and you can drive the battle to the coastal provinces. Then you use the tactic of boatbombs - you put a weak army as a bait, the main forces are waiting in the sea. Then when opponent attack, you drop the armies and smash him. Morale penalties do not affect efficiency, so you crush it's armies bu small pieces. This tactic, when used wisely and with good retinues allows to win war even when the opponent have more troops, but they are spread across a country, like HRE or Byzantium.
Best Answer
Thanks Naumfak for pointing it out that I was mistaken about single county conquest. That is indeed odd. However in case of a holy war or a prepared invasion the following stands true. If a reformed organized religion's (i.e. Islam & christianity) independent realm gets invaded by Pagans (e.g. Vikings), fellow Christian/Muslims will offer to join the war on defenders' side even if they are not allied to any belligerent party. For example if I as Petty King of Mercia get attacked by great Heathen army from Scandinavia, Other petty Kings, chiefs, counts, dukes, Kings can offer me assistance if they belong to the same religion as me.
Or if that NPC had a claim on that country, he could start an independent war, being hostile to all parties of the older conflict. For example, William the Conqueror & Harald Hardrada invading England at the same time.