I think Crusades are hardcoded to automatically attack any army that tries to interfere with them, no matter how well-inentioned their would-be saviors are. (I'm obviously excluding cases where you're allied with the Crusading faction.)
Of course, I can't say with certainty that that's the case since I'm no programmer, but it's consistent with my experiences thus far. Unless you're allied to the Crusading faction, it appears the Crusade will automatically attack you if you try to help it out. (Yeah it kind of sucks, but what can you do?)
Incidentally, the same principle seems to apply to Jihads as well. In one of my Egyptian games, I tried to help the Turks out in Rum (which they were trying to take back from the Byz via Jihad), only to have both them and the Byz gang up on me. I was forced to retreat, as I'd dispatched only a relatively modest army of ~800 men.![]()
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