I'm not sure this is the best place to post this, and I apologize if it's not the right place.

If there are troops of your ally in your current alliance, and you leave your alliance, what happens to your ally troops? Do they just die, without damage, or is there a war at the place where you both have troops?

They actually stay at the place, what happens after is rather poorly explored territory I fear =(

Many different situations..

If your units are on one of your former alliance member/s outposts or colony, You cannot attack from their origin. You can only move them to a friendly destination i.e your outposts/colony.

They won't be attacked unless the outpost they are present on is actually under attack.Hence they will instead defend the outpost in case of any incomings(even if not present in same alliance).