Sacred Haven of Debauchery

They probably split up.

I thought the same thing.

Looks like my ally is planning to conquer the entire galaxy. They attacked another empire that I was friendly with.

I also like to use similar tactics. If opponents have formed defensive pacts, but not all of the pacts overlap (for example, empires X and Y dislike each other, but both are in a defensive pact with empire Z), then I attack empire X/Y (even if the real target is Z) and invite the remaining one to join the war on my side.