Let's not be hasty. Declaring war on another galaxy is a bit extreme, don't you think?
I think a lot, but I don't think that.
The Dark-Matter Entities we have observed here at the core are preparing to destroy our Galaxy.
We are preparing to stop them. When we do so, we have to assume that they will somehow talk to their "leaders" or whatever in Andromeda.
They declared war on us some 100,000 years ago. We are about to announce that we've finally noticed.
But that's not the same thing as declaring war.
Well, it certainly isn't a wedding announcement.