We've been at war for centuries. The Empire and Insomnia aren't exactly friends.

[It was all because the Empire wanted that crystal kept hidden away in the palace. Once the Empire gained control of the crystal, the shield would drop over the city, and Niflheim could take Insomnia. Yet, that had indeed happened in Ardyn's timeline. Insomnia had fallen and now he had gained control of the crystal.]

It does come in useful in the right hands. Now, tell me, what is San Francisco like?