Ramping up to War (Homebrew Campaign Advice)

[Burley]

I ask for a lot of help with my homebrew campaign. I sometimes even get it, though I didn't get any help when I asked on my primary forum.

Anyway, I'm running a homebrew Oz game. Broad strokes: Humans are an invasive species which gained a foothold in Oz thanks to the Do'Rothee's accidental magicide and their use of Alchemerald, an artificial crystal that siphons Oz's natural magic for human usage. The Emerald Empire completely controls the Mun Country and controls middling territories in the other three countries (The Qua, The Vinkus and The Gillilands).
The party recently raided the Munkin governor's mansion, killing a half-dozen Imperial personel (including a Alchemerist's apprentice, while his master was bound and gagged in his bed), before descending into the UnderOz (similar to Underdark) to meet with Nerossa the Slumbering Elder of Subjugation. They will most likely be UnderOz for about a week in-game-time (one or two sessions, depending on if they wanna explore or leave).

While they're underground, the Empire will be mobilizing. The party has been propaganda'd into being the leaders of a Munkin resistance force (which they only really did one food gathering mission for). They'll emerge top-side to Imperial blimps, Yellowbrick Road blockades and more Alchemerald tech than Oz has ever seen. Going full-on Girl Genius clanks and death rays.

I want the world to be a shock when they emerge. I want it to be obvious that things are different so they know the "get on a magic carpet and fly for six days" option doesn't exist, anymore. But, an excessive show of force could (should?) signify a ramping-up of war efforts and I'm not quite ready for that part of the campaign.
I'd love some advice here from anybody who has run geopolitical/wartime campaigns. I'm used to running the "there's a bad-guy badding over there" kind of game and I'm not sure how to pace the impending magical war.

[Naaman]

Seems like the first thing you'll need to do is change the attitudes of the majority of NPCs to a default of unfriendly. It may also help sell the feel if they encounter a rare few who are actually helpful, since now they see them as champions of their cause (due to the propaganda). Depending on the play style, you may even want to have one or a (very) few of the seemingly-helpful NPCs be spies trying to embed themselves with either the PCs or one of the opposing factions, using the PCs as their "in."

In effect, the NPC population acts as a de facto nationwide surveillance system. You may want to set up a "hotline" for ratting out the PCs. It could be some kind of magical or emerald-tech device that has been placed at every tavern, inn, and tackle shop that allows anyone to sound the alarm if they see the PCs or know something about their whereabouts. It would be important to include instances of false sightings (which would probably be the majority of reports) so as to maintain verisimilitude.

You may also consider imposing a curfew, enforced by the blimps and other patrols.

Just a few thoughts to get things started. Hope it helps.