Respectfully, the land is white and therefore free for the taking.
Also, I don't need Africa, I'm just after Gibraltar.
I think that it goes without saying that as an ally of mine, you would only benefit by my controlling Gibraltar before some potentially hostile one does.
Lastly, I've made my hopes for peaceful and beneficial annexation clear. As a fellow ally of Atsina, it stands to reason that you should support this as opposed to a military occupation by a potentially hostile power.