With the way an Alliance with another empire works, when you have a fleet in system and an ally has a fleet or world also in that system, you have the option to either assist in an attack or in defense respectively if a combat activity comes up - or of course the option to ignore it.
So I noticed when doing a playthrough as the Phidi, I was allied to both the Ashdar Colonials and the Orthin. Those two empires were at war between each other at that time. An Ashdar Fleet came to stomp a Orthin world (where I also had a planet and fleet in that system). In this particular case, I was only given the option to assist in the attack, not assist in the defense of the planet. Do the attack options carry more weight with the AI? Also, if I ignored the attack, I wasn't prompted to assist in the defense.