For article on Crusades go here : Crusades
Jihad is a Muslim concept believed by most Muslims, be it Shi'a or Sunni that it is a stuggle to fight against anything that is considered dangerous or decadent by a Muslim. In Crusader Kings II, Jihads are very similar in terms of aims and means to the Crusades done by Catholics, although JIhads are different in which a Muslim ruler must follow his own leader, be it Shi'a or Sunni. Jihads tend to aim for entire kingdoms like France or Anatolia and gainings will be given to any Muslims who held the title or those who contributed the most during the Jihad.
In Crusader Kings II Edit
In Jihads, every once in a while, most will happen after a Crusader is done and if the crusade is won, then the caliph, usually the Abassid caliphate, will issue the order to Jihad. They usually target Christian nations, but sometimes they will target pagan nations as well. As with crusading, any rulers not joining will get an opinion penalty of -15 while those who joined will gain a +20 opinion gain from the caliph. Edit
The Caliph will usually start alone fighting the target nation, but rulers are allowed to join in (or forced to) to increase the fighting force to fight the enemy. The few most earliest targets for a Jihad (if started in 1337) would be Andalusia, held by Castille; Aragon, or Jerusalem if a Crusade took Jerusalem first.
Also note: anyone not a Sunni Muslim can be a target of a Jihad, even of you are a Shi'a. So if you are the Khagan of Ilkhanate and you are Shi'a, the Sunni Caliph can issue a jihad against you.
Like Crusades, Jihads have contribution % that works exactly like the contribution % for crusades. The nation with the highest % of contribution can reap all the rewards of a Jihad. To increase the %, it is easy to just fight armies with armies, whether you win or you lose, you will gain significantly higher % rather than just sieging holdings here and there. Note that capturing provinces increases Jihad warscore while battles increases % of contribution, so it is very wise to devise a strategy that can make sure both the warscore and the % increase on you.
Also note, like Crusades, if a ruler holds the contested title of the Jihad, then he will gain the target of that Jihad, no matter how high a ruler's % of contribution.
Mujahid/Mujahideen Trait for MuslimsEdit
When a Muslim goes on a Jihad, and when he leads an army to the target kingdom, a ruler can get the Mujahid trait. This trait is the equivalent to a crusader trait that Catholics have. This trait allows the increase of marshal skills by +2 while also increasing other Muslim's opinion on you by +10, so this is a very profitable trait to own.
To get this trait, one must lead an army to a target area, no matter how big the army is. Also, if your heir is an adult (16 years old minimum), you can position him in your army to gain that trait, even if he is not a good general. If that is the case, then wait until he got the trait, then replace hiim with a more competent general. Don't do the said tactic if you heir is a very good general.
The same goes with the Crusades, women cannot get this trait unless the console command is used.