So I'm playing as a count and I start a war outside the realm. I invite my brother in law into the war and he accepts, so far everything is peachy. However, much to my dismay the duke I am vassal to joins a revolt against the emperor, putting me and my ally on opposite sides of the fight. However my brother in law is a good ally, despite us being at war he sends troops to the holdings of the guy I'm attacking. The troops arrive... and start sieging the holdings I've already taken, returning them to their owners control. Somewhat less then the ideal ally, no?