Basic Trope: Aliens invade Earth primarily or entirely to use the planet's water.
- Straight: An alien civilization is at risk of extinction because of dwindling water supplies, so they invade Earth and fight a war with the Earthlings over control of the water supplies.
- Exaggerated: A group of aliens are thirsty, so they travel to Earth and kill every living thing in order to get ahold of a few glasses of water.
- Downplayed: The aliens are not at risk of extinction, but Earth's water supplies would be a great boost to their industry, so they take over Earth (or at least a nation close to water) to gain control of the water.
- Justified:
- The aliens' telescopes are not much better than ours, so they do not notice that there are plenty of water-rich worlds in the Solar System besides Earth.
- The aliens literally come from Mars, which In-Universe is a dying but inhabited world. Earth is the only other inhabited planet in the Solar System and the only one possessing large quantities of liquid water. The Martians lack the means of interstellar travel.
- The aliens lack the necessary industry that would be required for orbital water harvesting and filtering rare elements from the water.
- The aliens are assholes who don't want to harvest water if there is a perfectly serviceable planet where you can get it anywhere. Oh, there's "humans" in it, who will fight back if we come to take a little water from them? Fuck them.
- Water may be everywhere, but it's not just water that the aliens want...
- The aliens lack space travel - they just happened to find a Portal Network so earth is the only water rich world they can invade.
- Inverted: Humans invade an alien planet to steal its water. Or aliens invade with the purpose of flooding the Earth.
- Subverted:
- The aliens claim to be after water, but it's a trick to mislead the humans; they're really after another resource, like minerals.
- Thinking the aliens are after water is a result of a translation error. They want /bodies of water/. Specifically largely aquatic worlds to colonize. Their own water consumption needs are supplied by space mining, it is the habitats they want.
- Aliens want to take the water but not for their own usage. They are simply paranoid that leaving water lying around for too long could result in more alien life arising which could be a threat to them later.
- Double Subverted: But the aliens also need Earth's water, for sustenance and/or industry.
- Parodied: The aliens are so mind-numbingly stupid that they went past countless ice filled worlds and comets because they somehow didn't know that ice melts into water.
- Zig Zagged: ???
- Averted: There is no alien invasion. Or the aliens are not seeking Earth's water.
- Enforced: It is an advertisement for a bottled water brand. They need to make water look particularly valueable.
- Lampshaded: "Why are the aliens coming here for water? Surely there are water-rich planets that aren't crawling with hostile sentient natives?"
- Invoked: ???
- Implied: Every world targeted by Planet Looters is shown turned into a desert.
- Exploited: ???
- Defied: Aliens steal all of the water on the Solar System except Earth's.
- Discussed: ???
- Conversed: ???
- Deconstructed: The aliens notice that the moons in the outer Solar System have lots of water, so they decide to extract water from Europa and never bother with Earth.
- Reconstructed: The humans also want to colonize and/or exploit the natural resources of Europa, so there is conflict with the aliens after all.
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