So looking into this, here is what it looks like to me. Correct any errors of understanding, please.
Redhaven was in Winkamus, Highmarch's region. Highmarch and Vix Tiramora were moving into Winkamus. Vix is at war with both Redhaven and Highmarch. Redhaven and Highmarch are at peace with each other.
The first part of setting up battle sides is choosing the defender. In this case, no one from Highmarch or any of its allies/federations were in Winkamus. So for defenders, the game selected Redhaven for having troops in the region (a bit more complicated process, but simplified explanation). Once the game selects defenders, it begins adding allies/federations and enemies. Since Highmarch and Redhaven were only at peace with each other, Highmarch didn't join the battle as Redhaven's allies/federation.
So in the end, Highmarch got Redhaven to defend Highmarch's region from their mutual enemy while Highmarch watched.
