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...