How to garrison my provinces.

[Martok]

There is a civil war if a crusade province falls into enemy hands? Hot damn! I just crusaded to a city on the north coast of Asia Minor and took it, so what if i lose it now? Civil war?

What Bretwalda is referring to is losing the province your Crusade originated in.

For example: Let's say you launched your Crusade from Wessex. If another faction should happen to invade and conquer Wessex before your Crusade reaches and conquers its target province in Asia Minor, the Crusade will disband and automatically fail--thus causing your king to lose massive influence and possibly trigger a civil war. In other words, don't launch Crusades from your border provinces (since they're vulnerable).

Losing a province taken via Crusade doesn't harm you very much in and of itself, as the surviving Crusaders are simply "disbanded" into a regular army. Your faction leader does lose influence, however, for every 2 provinces you lose. (Likewise, your king also gains influence for every 2 provinces you conquer!)