The idea is interesting. Technically naval ships, but not really what we think of. It's already an interesting premise.
Chapter 1: I don't think the INPF or any international coalition can be founded in less than a week, especially in 1939 considering the political situation. The first week alone would be finding out who the hell attacked them, what the hell the Belles are, and so on. I'd wager by either November-December 1939 or even in 1940 the INPF would come in. Granted I am not extremely well versed in international politics of 1939 so I'm just guessing. (Note: I see that almost immediately after making this observation the Americans are in agreement that this has gotten together extremely quickly)
Chapter 2: In regards to Ferdinand, excuse me what? It doesn't make much sense that she's already manifested when so far all we know is that Belles only manifest in proximity to Morganas, unless I missed something. Even reading the excuse in the second chapter I'm still a bit unsure if it works
Chapter 2: I wouldn't mind someone with better airship knowledge than me to tell me if the weaponry of her is too heavy or works out ok. Just seems excessive to me, but I know barely anything about airships so feel free to ignore me.
Chapter 3: To draw more attention to the comment on the chapter itself, I don't think that's how you correct a dirigible.
Chapter 3: The belle senses thing is interesting. To see it more fleshed out in the future may not be a bad idea.