How to stop crusaders deserting?

[sapi]

You need to move a certain distance to the target each turn.

Having agents with the army will prevent you from doing this.

Some paths, such as that from spain, will not count as having moved towards the target (moving south through the iberian peninsula, for example, is not moving towards the holy land) and the best way to get around this is to travel by ship, taking advantage of the cheap crusader merc ships.

It's a good idea to land each turn, though, in case you get caught in a naval battle.

[DrZoidberg]

You need to move a certain distance to the target each turn.

Having agents with the army will prevent you from doing this.

Some paths, such as that from spain, will not count as having moved towards the target (moving south through the iberian peninsula, for example, is not moving towards the holy land) and the best way to get around this is to travel by ship, taking advantage of the cheap crusader merc ships.

It's a good idea to land each turn, though, in case you get caught in a naval battle.

I am moving at full speed every turn.

[HoreTore]

Well, you have to ask yourself; are you moving the quickest way possible to get there?

That doesn't mean "quickest way with ships" or "quickest way by land", it means the quickest way combining both. For spain, the quickest way to the holy land is NOT walking through the desert. The quickest way by far, is to sail directly from valencia to jerusalem, moving the max distance each turn of course, and not getting bogged down by enemy/neutral ships. You can easily detour a little, but until you know how to launch a succesful crusade, I suggest you don't, and go straight for the target.

To this date, I haven't had a single crusade desert, except when I've been beaten at the target, or deliberately taken a detour knowing parts of the army will desert.

[steev]

If you're moving through the desert and there's no road, you're going way too slow and all your army will bug out. If you've got a paved road its ok, otherwise forget it. go by ship.
usually you don't want to go all the way there in any case. Just far enough to snatch a few rebel or muslim held cities. no point having a foothold in the Holy Land when you've got nothing in between. But be careful how you pace your moves when you stop to sack a city - if you end your turn too close to the city you won't move enough the next turn before you besiege, and troops will bug out.

[Jasper The Builder]

This has happened to me on a couple of occasions also.

I came to realise that the generals i had sent on the crusades lacked in loyalty and authority. When ive taken a Prince or a Faction Leader ive never had this problem. Like with all things within the game of TW its not always one problem that makes these things happen, I agree with the above also as being one of the major things you must do whilst crusading. Making sure the route you are taking is the fastest and most logical way, Having a number of scouts and spy's pathfinding their ways out in front and generally being totally professional about hoe you go about doing it.

Its always a good idea also to take a number of priests along with you, To rally the soldiers hearts.

[sapi]

I wouldn't take any priests with a crusade; rather send them ahead.

Keeping agents in a crusade/jihad stack removes the added movement bonus that they get and makes it impossible to move far enough each turn.

[Sentinel]

There is a trick (exploit? / cheat?) you can use which enables you to move a crusading army in any direction without suffering any desertion.

As a test I took a HRE army that had just starting a siege of Baghdad and marched it all the way back to Antwerp without suffering any desertions

Spoiler Alert, click show to read: 

All you do is move the army in the direction you want, until you have no movement point, left then click the "leave/join crusade" button twice. Once to leave, then again to immediately rejoin.

There is usually a time limit starting when a crusade is called of about 10 turns during which the above will work. After this limit has expired you can leave the crusade but will not be able to rejoin.