battle question - did not assist allies

[Azerax]

Greetings all,

My character is in a region with an Ally.  A realm that they are at war with, but we are at peace with, attacked.  We did not join the battle.  Our units were sent to defensive.

This same situation happened about 3 months ago, but we did join the battle, and relations have not changed. (maybe someone had aggressive settings?)

Any idea why we wouldn't join the battle this time?

[Anaris]

Insufficient data.

In general, this is the correct behaviour. You should never fight in battle against a realm you have a peace treaty with unless you are murderous (and in that case, it will only work if you are attacking; you will still not help your ally defend against them).