I really hope the devs give us no heads up whatsoever. I dont mean foregoing adding lore on the subject or intelligence on the forces themselves via the citystate papers, but do something similar to the arcanist that just started popping up and then vanished as soon as he appeared on the scene. It would be great if everyone was just exploring on chocobo-back, completing leves, quests, missions, or just sitting around, and then all of a sudden...the event starts.
The surprise of the Garlean skirmishes in Thanalan, or the Arcanist really helped bring back some hype/sense of adventure behind the game, and i'm really hoping that they end up pulling something similar off when this occurs. Even if its just Garlean Airships spawning on the far horizon, troops just appearing out of nowhere in all areas near city states, along with a consistent music theme change that took place over a few days- a week. This would probably satisfy those that want more immersion, a challenge, or just something cool to watch/do.
Think of it as how Zanarkand citizens were surprised when Sin attacked out of nowhere...or the Beastman assault on Tavnazia in the FFXI opening video. If you're feeling old school, the Red Wings attack on Mysidia or The bomb ring explosion in Mist.