When does an Ally Fight with You?

[Eternal Champion]

Theres some guy with a big cresent moon,
"i cant build him only seen enemy's send him to get my land i had taken from them so i dunno what its called"

That is a Jihad, it is kind of the Muslim version of a crusade. It can only be used to retake land once held by that faction. While that is somewhat limited in scope the offset is they are rather cheap to build and you can have many at one time.

[Zarax]

Multiple Jihads is a very cheesy tactic, though in some occasions it turns out to be quite useful especially when using some mods...
For example i love playing as the Turkish in early as after you vanquish the Eggys you don't have a navy, ending with your coastal provinces exposed to naval invasion...
After a few game tweaks i did i find myself building jihads in order to keep tripoli and antioch from being overrun by the Italians who sends at least one stack per year...

[Pericles]

It's pretty cool when an ally fights alongside you.

In one of my VI Campaign games as the Irish, I had landed in the British Isles and attacked the Welsh. To my pleasant surprise the Saxons also attacked the Welsh.

When the battle started I watched as the Saxons (with flags flying) moved en masse towards the Welsh, while I started a flanking maneuver. It was like a scene right out of Braveheart as our armies moved over the rolling hills towards the waiting Welsh armies.

That was one battle I saved as a replay to watch again...