Something I noticed.

[NodachiSam]

Just a note but I noticed that crusades that are going through your land are obliged to help defend the province probably regardless of who is attacking. I was playing western Roman in the Fall of Rome MTW mod and a barbarian crusade/horde passing through what would become burgundy helped with defend the province from the Franks. (playing western Rome is rough) I'm pretty confident this would be true in any version of MTW. It's just something I never noticed before.

[Foreign Devil]

Is it an allied army or just another army? Is there even a difference? Don't remember... I just started playing again after a looooong break

I remember in Shogun fighting battles with three armies on the feild, trying to manuver so you didn't engage until the other two had been going at it for a while. Those battles were always a lot of fun, but I can't remember it ever happening in MTW.

[Weebeast]

Well, I had something like this when playing XL as The Crusaders. I had Constantinople and it was raided by either Cuman or Bulgarians (can't remember). French crusade was present and it helped me defend my province. I think it helped me cus French was my ally but I'm not sure. It seems that way anyway.

[matteus the inbred]

i had one a while ago in VI where there was me (Welsh), Mercians and former Saxon rebels all going for the same province, and all fighting each other. i smashed the rebels off the field with unintentional Mercian help and then narrowly defeated the Mercian force when their advance left them stuck at the bottom of the valley, but it was too late to extract my army to a decent location when the Mercian reinforcements arrived, and the last of my men went down fighting. sent a fresh army in next turn and took the province, went on for a 60% victory, but that battle was great, really tense. i made the classic mistake of being one of the two armies that start fighting first, rather than waiting and destroying the survivors...

i've never yet been helped by a Crusade though. i've had my crusade helped by Byzantines, but not the other way around.

[NodachiSam]

I think a crusade has to be neutral or allied to help you. I guess the thinking is that they too are occupying the province. If I'm not mistaken they were actually helping defend from a another crusade though that ones was deflated severly after trying to get into another province of mine.

Other than said example I think I've only had it once or twice where 3 different armies on the field, two of whom weren't allies. Usually the third force are rebels. It was more common in Shogun.