I don't know, it certainly sounds like a move to gain strategic advantage to me. There's really no other reason for doing it. You do it so:
1) You can hold onto the city.
2) You get a base of operations that allows recruiting that's not 800 miles from Luria for further actions against Morek.
Those are purely strategic considerations for the express purpose of gaining an advantage during a war. It would be one thing if they held it for the duration of the conflict and then later seceded it to create a new realm. But to do it right away in the middle of the war, I don't know. Seems like the primary practical result of that will be to give Luria a huge strategic advantage via secession.
There's a big difference between seceding half your contiguous realm to gain a second recruiting center on the other end of it—or moving the capital so that your recruiting center is there—and carving out a forward base in enemy territory, a long way from home.
