For article on Jihad go here: Jihad
The Crusades in the Middle Ages were wars that were ordered by the Pope of the Catholic Church for Catholic's to fight the infidels in the name of Christ, and those who succeeded will be given remission for sins, in order to gain access to heaven. In Crusader Kings II, the Crusades are different in that their aim is to conquer holdings within a specific de jure kingdom, with the gains being granted to the catholic ruler holding the kingdom, or, if the kingdom is not held by a catholic, to the ruler contributing most to the war.
In Crusader Kings IIEdit
Every once in a while, the Pope will order a Crusade to the Catholic nations to fight an infidel, usually to a Muslim nation. To a Catholic nation that does not follow the Crusade order, the opinion of the Pope will usually be decreased by -15 and to those who follow, will get an increase of +20. While early in the game as in 1070's crusades might also be ordered to take a pagan nation.
When a Crusade is called, it is highly recommended to get allies to join, especially if the target of a Crusade is extremely powerful. For example, if the target is Egypt in the 13th century, 13th century France will need some help from other nations like Scotland or the Holy Roman Empire to assist in that crusade. Or if your nation is very powerful with massive levies ready to be deployed like the Holy Roman Empire, you can go solo and defeat the target alone.
Targets of the crusade usually have done things diplomatically that ruins the politics of a certain nation. For example: If the Sultanate of Rum defeats Georgia and takes Derbent, then it is possible for a crusade to happen so Derbent goes to Catholic hands.
Also, note that any heretic nation can be a target of a Crusade, so keep Religious authority high to not become a heretic. The Orthodox Church however, will not be subject to that because the Eumenical Patriarchy never issues a crusade.
What to do after a CrusadeEdit
If a nation/you finished and won the crusade, you will get the land that is targeted and the de jure duchy that is attached to it. Beware however, you will gain holdings that is not a barony also,make them a new vassal so you don't have a rebellion later, and if the duchy fell to other vassal's hand, make sure you keep them happy because the long distance between them and you can cause a problem.
Note also that the Sunni Caliphate works similarly like the Papacy. If you won the crusade, there is a chance that the Caliph will issue a jihad against the nation who won the crusade. Jihads are similar to a Crusade in that it orders either Sunni or Shia nations to wage war against the infidels, most likely Christian nations, both Catholic or Orthodox. Either of the two Caliphs can and probably will call a jihad, so you need to be prepared for a counterattack.
If a crusade is successful, the target of the crusade goes to the nation who has the highest % contribution. A few % points can mean the difference between you getting awared a shiny new kingdom, and some other nation getting the titles.
The most powerful way to gain contribution is to fight enemy armies. The more you fight, and the more soldiers who die in battles lead by you, the higher your % contribution climbs. This even counts for battles that you lose. Seek out enemy doomstacks with your own and engage early and often.
You do get some contribution % for capturing provinces, but it is very small compared to the points you get for fighting active battles. Ironically, it is the capturing of provinces that increases the crusade warscore the most, while winning individual battles does little.
The ideal situation has you hunting down and crushing the enemy with your army, while your crusading allies park their armies in various provinces and slowly capture territory. Your allies will generate the warscore needed to win the crusade, and you'll get the contribution % needed to be awarded the kingdom.
Note that if a crusade is called on a kingdom that is already owned by a catholic nation, that nation will gain all land from the crusade, regardless of contribution % scores. So if any catholic claimant exists.
The Crusader TraitEdit
The crusader trait is a very desirable trait for any character to have. Not only does it give you +2 marshal, it increases your relationship with any church characters by 25 as well as increasing it by 30 with other characters who have the crusader trait. This includes the pope. For this reason alone, it is recommended to join all crusades called, even if the pope is likely to lose or you are unlikely to be the top contributor.
If you have a male heir who is 16 or older, it is advised you bring him along for the crusade as well. Even if he has a lousy marshal skill, participating in a crusade will net him the crusader trait...which will be very useful when you eventually take over as him. It is also handy to try to get all your vassals to have the crusader trait since the bonus of 30 to relations for characters with the same trait as well as the 25 for church characters can increase relations by up to 55 for church characters who have the crusader trait.
If your ruler and/or heir has a lousy marshal skill, simply make them generals and land your army in a province that is part of the crusade, but not near enemy armies. After a month or so, an event will trigger that will net you the crusader trait. Once you and/or your heir have the trait, you can safely replace them with more competent generals and start crusading properly. That said, if your ruler/heir does have a high marshal skill, keeping them as generals can net them a TON of piety when they win battles during a crusade. This is only recommended if your ruler is a legitimately competent general.
Note that simply joining a crusade is not enough to gain the crusader trait. A character must be leading troops stationed in a provinces that is part of the crusade's target kingdom. For this reason, it is almost impossible for female characters to get the crusader trait unless you have some kind of Cognatic inheritance and your female ruler is leading troops during the Crusade.