Remilitarized Zone

Stand to attention, player! You're not in Green Hill Zone any more! This videogame setting suddenly has a lot of guns, bombs, tanks, warships, soldiers, barbed wire, trenches, smoke, and mines, often in stark contrast to the rest of the game. Often seen in platformers to mix up the usual mix of generic level types.

The music will generally switch to a more sinister theme to warn you, if not a straight out military marching tune.

Don't be surprised to see Schizo-Tech, Super Soldier enemies, a Military Mashup Machine for a boss, and for the battlefield to resemble Mordor.

Examples of Remilitarized Zone include:

The tank parade and warships of the Dark World in Super Mario Bros 3.
The final regular level of Gnasty's World in Spyro the Dragon, and some areas in following games. (including the war betwen the Breeze Builders and Land Blubbers). Also the (ironically enough) Peace Keepers homeworld.
The Sonic the Hedgehog games use this every now and then.
- Huge Crisis Zone in Sonic Rush Series.
- Hidden Base Zone in Sonic Advance Trilogy.
- Flying Battery Zone in Sonic 3 and Knuckles.
- Sonic Adventure 2's Prison Island portion is big on this one. Metal Harbor, Weapons Bed, Security Hall, Iron Gate, and Prison Lane all fall under this trope. The first two take place on the island's G.U.N. naval base (Sonic even climbs up an ICBM launch pad and hitches a ride on the missile); the other three are more accurately within the The Alcatraz trope. They are in a top-secret military-level prison, however...
- Most Shadow the Hedgehog levels have this in one way or another. Central City, Westopolis, G.U.N. Fortress and Air Fleet are probably the most applicable to this trope.
- Sweet Mountain in Sonic Colors... and it's made out of FOOD.
The Rikti War Zone in City of Heroes. Also the Shadow Shard, though not as well known for it...
Warcadia in Folklore. Particularly noticeable since you head there directly from the Faery Realm, a fairyland filled with friendly little elves, breathtaking architecture, and lots of big, green trees. Whereas Warcadia is filled with monsters based on soldiers and WWII-era weapons, craters, burning buildings and at least one Military Mashup Machine.
When the Army comes in Half Life, and some places of Half Life 2.
The War levels of Mischief Makers had lots of tanks and bombs. You even rode a missile at one point!
The very first level in Psychonauts, Basic Braining. You're in the mind of your coach, who is obsessed with the military, and thus everything in his mind that you can see so far is war, war, war. Explosions, war-themed figments, minefields, cannons, camouflage. Even the trees are made of bullets and the rabbits are wearing soldier's helmets.
The Kingdom of Anger, Volk, from Klonoa 2: Lunatea's Veil, a kingdom consisting entirely of people who do nothing but fight all the time.
The War chapter of Conker's Bad Fur Day
Wario's Battle Canyon from Mario Party.
Technoir in Jazz Jackrabbit.
Super Mario Galaxy has several examples of this, and becomes scrappy when it comes time to collect Purple Coins in them.
Napalm Man from Mega Man 5 has this for the second half of his stage.
The aptly named Planet Batalia in Ratchet and Clank (the first one).
All Metal Gear games feature these prominently, of course.
- And thus, Shadow Moses in Super Smash Bros. Brawl
Sometimes Inverted in Metal Slug: while the game is about (at times wacky) war, some areas have little to no killing machines trying to kill you. Of course, there are always something else trying to kill you...
Military base in Painkiller.
Moon Kronor in Serious Sam II.
Planet Norion and Pirate Homeworld in Metroid Prime 3.
Ichor in Jet Force Gemini.
World 2 ("Combat Island") in Rainbow Islands. With cute, cartoony-looking bomber planes and tanks, no less.
Pretty much ALL the levels in Heavy Weapon are like this.
The Halberd in the Kirby series. Meta Knight's airship is probably the biggest example of Mood Whiplash in the series that doesn't involve a Cosmic Horror.