Hostile Terraforming

Imagine you are a leader of an alien race currently at war with another race, and your natural habitats are drastically different. How would you perform a hostile takeover of a planet?

By making it so the planet is now inhabitable by you and not by the natives, of course! Converting the atmosphere, reshape the land and sea, overrun the native ecosystem with one more suitable - whatever is necessary to make the very ground your enemies tread on no longer their ally.

A specific context for Terraforming, and if it's an Earth-like world being changed for to another ecosphere, the term would be "xenoforming". Depending on methodology, it can involve Grey Goo, Gaia's Vengeance, or Meat Moss. Due to the destruction, this is a Class 4-5 Apocalypse How, depending on how much of the planet's multicellular life is replaced.

Examples of Hostile Terraforming include:

Anime and Manga

Lord Slug wanted to freeze the Earth because of this in the fourth Dragonball Z movie. "De Terraforming" was discussed in the Dragon Ball Abridged movie version.

Comic Books

The first issue of Sillage/Wake involves an alien who intends to terraform a jungle planet to suit his species, although in this case he's unaware that there's one sentient being (the human girl Navis) and lots of semi-sentient animals already living there.

Film

The Arrival. Aliens are causing global warming in order to kill off humans and make the planet more comfortable for their kind.
They Live! used a similar premise, with aliens "turning our world into theirs".
Battle for Terra - The humans sends a machine that can convert the air on Terra to be human breathable in seven days. This would leave the air unbreathable for the Terrians.
The Trollenberg Terror is about aliens' native air slowly enveloping a mountain and later the surrounding area.
In Man of Steel, Zod and his people attempt to "Kryptoform" Earth.
The Genesis Device from Star Trek II: The Wrath of Khan. In Star Trek III: The Search For Spock the Klingons expressed concern that it would be used for hostile terraforming, and with good reason -- it is very easily deployed as a weapon and it works on a timescale that is virtually instantaneous compared to typical terraforming efforts (thus can't be stopped).

Literature

In the original War of the Worlds novel, Martians use areoforming -- in the form of the "red weed" -- as a weapon, essentially. Possibly an Ur Example.
Last and First Men, the Fifth Men escape a dying earth by terraforming Venus. Halfway through the process they discover that Venus is inhabited and oxygen is lethal to the natives, they keep at it anyways.
War Against the Chtorr features this prominently.
A specialty of the Yuuzhan Vong in the New Jedi Order.
This is the long-term plan of the alien invaders in The Tripods series.
The Star Trek Expanded Universe book Dark Mirror mentioned that one of the earliest missions of the Mirror Universe Captain Picard had been the "prejudicial terraforming" of the Ferengii home planet, which "cleansed" the planet of all life.

Live-Action TV

In Quatermass II an alien vanguard takes over selected humans so they can build a chemical plant to make an atmosphere that will support their kind of life, and kill off all terrestrial life.
The Star Trek: The Next Generation episode "Home Soil" featured a Federation terraforming project that was doing this by accident.
Star Trek: Deep Space Nine: Sisko used a specialized warhead to poison the atmosphere in such a manner that it would be uninhabitable for human life, but functional for Cardassian in an attempt to force Eddington to surrender. Though the Maquis did pretty much the same thing (except rendering it uninhabitable to Cardassian life but safe to human) to multiple Cardassian colonies.
In the Stargate SG-1 episode, "Scorched Earth", an alien vessel was terraforming a planet inhabited by Human Aliens into something hostile to their form of life. Unlike most of the civilizations SG-1 encounters, the aliens need a very specific environment to survive, meaning they can't easily be moved to another world. Eventually it's resolved when it turns out the terraforming ship has information about said aliens' home planet.
Doctor Who, "The Sontaran Strategem". Weakened by their eternal war with another race of Scary Dogmatic Aliens, the Sontarans hatched an uncharacteristically circuitous plan to bathe the earth in gas which is poisonous to native life but nutritious to themselves, so that they can use it to spawn more soldiers.
The Andromeda episode "Point of the Spear", the Pyreans (aliens that live in Venus-like environments) tried to forcibly pyroform a Commonwealth world. A large battle breaks out, and to prove that the Commonwealth is not one to be messed with, Dylan orders the planet's destruction via Nova Bomb.
One episode of the new The Outer Limits had this with a new model of car that would poison the atmosphere for infiltrating aliens.

Tabletop Games

Metagaming's MicroGame 12 Invasion of the Air-Eaters. An alien race invades and uses Atmosphere Converters to change Earth's air to sulfur dioxide, which they can breathe it but we can't.
Warhammer 40,000
- "Tyrantforming" is the first stage of devouring a planet by the Tyranids - the spores dropped onto the surface merge with local plantlife, turning it into Hungry Jungle - rapidly draining the ground of all nutrients. The Tyranids then devour the plants.
- Orks are described as "an invasive ecosystem by themselves", and shed spores to generate new creatures, which will grow to be squigs, snotlings, gretchen or full orks depending on the conditions. With enough orks, they create their own flora and fauna, and the whole ecosystem is "orkoformed".

Video Games

The Command & Conquer Tiberium series centres around humanity adjustments to a world being transformed by the eponymous Technicolour Crystaltech - which not only convert the atmosphere and the ecosystem, but also extract useful minerals from the Earth's crust for easy harvesting when the invaders finally arrive in person.
Half-Life 2 in its released form contains hints of this - most notably, the Combine drain the Earth's oceans and use Xen species such as Headcrabs and Leeches as biological weapons. The original beta however went much further - a key location in the game was the "Air Exchange", which replaced the air with noxious gases breathable by the Combine, and which meant that all human characters in the game had to wear gas masks.
In Star Control II, Mycon Deep Children reshape planets they collide with, stimulating their volcanic activity to the point where it's comfortable for Mycons - obviously, since they enjoy temperatures above 600 Kelvins, it's less than comfortable for pretty much everyone else. Mycons don't seem to care.
In Spore, you can deterraform planets of an enemy empire to reduce the level of colony that planet can maintain (especially effective against enemy home-worlds, as it reduces them from thriving T-3s with extra settlements, to a basic T-2 with two settlements at best). Empires will regard this as an act of war. More in line with this trope, the Grox, Enemy to All Living Things, can be killed by terraforming their planets.
Phazon from the Metroid Prime subseries tends to do this. It turns out to be Sentient Phlebotinum that originates from the planet Phaze, which deliberatly seeds the universe with phazon meteors.
The Korath Clan in Galactic Civilizations 2 prefer to eliminate a planet's entire biosphere. They can then colonize the resulted toxic world.
All K-D Labs RTS games employ combat terraforming to some extent. All sides in Perimeter and two sides in Maelstrom can terraform the battlefield to create passages and so on. Both sides in Perimeter 2 (Exosus being waterborn, Comeback being landlubbers) and Hai-Genti in Maelstrom instead make parts of land inhabitable only for themselves.
In Lost Planet: Extreme Condition NEVEC is planning to terraform E.D.N. III in a way that would instantly thaw out the planet and kill the native Akrids and the rebelling colonists. Fortunately Wayne discovers an alternative that won't fry the colonists and by Lost Planet 2, ten years later, most of the frozen wastes of E.D.N. III have been replaced by deserts and jungles, and several new Akrid species are coming out of hibernation.
In Stars! races with Claim Adjuster trait can undo enemy terraforming using Retro Bombs and/or use Orbital Adjuster on a planet without conquering it, and Packet Physics races terraform planets they bombard with mineral packets. If environmental requirements of the sides are different (frequently, as those are 3 independent variables), it's de-terraforming for the target.
In Haegemonia: The Solon Legacy, high-level spies can reverse-terraform enemy planets. It's not permanent (the enemy can simply terraform it back if they have the tech) but as planet quality is the primary factor in maximum planet population, killing off a few hundred million colonists or at least inciting revolution due to massive overpopulation with a single spy can really wreck someone's day.
Age of Wonders from the first game on has various "Path of X" unit abilities, turning terrain into the type preferred by the unit. This usually benefit only this unit tye (e.g. Frost Queen has Path of Frost and Snow Concealment), as well as cities of this unit's race, and conversely removes terrain advantageous to the others.

Web Comics

White Noise had this happen in the backstory. The aliens who killed off everyone on Earth's surface then attempted to terraform the planet to suit them, resulting in a clouds of toxic Murk that roam the surface.

Web Original

Orion's Arm:
- Happened on occasion due to many early colonists preferring to adapt to new environments rather than terraform, putting them into conflict with later waves who would rather change the planet than themselves. Most notably when the original Martian Tweaks were forced to leave their homeworld when the atmospheric pressure and oxygen content were raised to intolerable (for them) levels.
- A similar thing happened on Venus, despite the extremophile tweaks wiping out the baseline colonists (the former were then banished from the solar system).
- However, the terraforming of Mars took long enough that a second clade of Martian tweaks emerged, and managed to halt the process at a level they are comfortable at.
- When Zarathustra, one of the first exosolar colonies, was settled there was minimal terraforming and the tweaked colonists went feral. A couple centuries later Jupiter Transsystems arrived, enslaved the natives, and set up terraforming stations that would have killed off the tweak population if they hadn't also awakened and pissed off the original colony AI.

Western Animation

Superfriends (1973–74). The inhabitants of the planet Solar Terrarium are moving the Earth closer to the Sun so it will become hotter and more comfortable for them. They are unconcerned that the increased heat and resulting climate change will kill most of the humans living here.
- In the 1978 version, the Legion of Doom allied themselves with Venusians, who needed them to increase the temperature of Earth and the vegetation so their invasion could succeed. Naturally, this is Artistic License in hindsight, as Venus is now known to have no vegetation at all.
In Transformers Generation 2, the Cybertronian Empire's modus operandi was to kill all the inhabitants of each planet and then mechaform the planet itself to make replicas of Cybertron.
In Justice League, the Imperiex attempted to do this in the series premier, and for Book Ends, the denizens of Apokolips attempted it in the series finale.
Oglethorpe and Emory of Aqua Teen Hunger Force tried to do this in the episode "Universal Remonster".

Real Life

Some scientists worry that terraforming Mars might kill any native bacteria that might exist on that planet since they think that they cannot survive under an oxygen atmosphere.