Worldbuilding Stack Exchange is a question and answer site for writers/artists using science, geography and culture to construct imaginary worlds and settings. It only takes a minute to sign up.
Sign up to join this community
What's the easiest way for your stereotypical aliens to destroy civilization from orbit? I'm taking "destroy civilization" to mean the destruction of all large-scale forms of governance, but bonus points for extinction.
Criteria: Either cheapest or least energy consumption. The aliens have access to anything that's roughly possible for a Kardashev II civilization. The aliens use a magic-plot-teleporty device to get here, but everything else should be as hard-science as possible.
As Samuel states, throwing a big rock at the earth will certainly end civilization....and everything else too. Short of interplanetary rock throwing contests, the easiest way to end civilization is to nuke each and every major metropolitan area. They're easy to find, just look for the big shiny lights on the planet surface.
If these areas go away then civilization ends. Banking no longer works, government no longer works, transportation systems no longer work (or heavily degraded), and worst of all the Internet will get really really slow.
You can't get paid because the bank that handled your paycheck isn't working and that's because the people who handle your paycheck can't get to work. How long would you last if suddenly ATMs and credit cards stopped working? What if your phone no longer routes calls or text messages don't go through? Would you panic?
Even if the attacks happened at night when most people are at home, only 2.6% of the work force actually works from home with any regularity which means that 97.4% of have to show up for work. (Admittedly the stats are US-centric but I believe are broadly applicable to any First World economy.) In Central Europe during the Great Depression, unemployment peaked at 20% and output dropped by 40%. Unemployment hit 70% in some areas of the UK. In this situation, unemployment is at best 50%. Nuclear attacks on every major metropolitan area have resulted in an unemployment rate at least double, double(!) the Great Depression. It took a world war to get out of the Great Depression but this time, there is no untouched infrastructure to help bootstrap the process.
Couple this loss of employment with 50% global casualties as 54% of humanity lives in cities and the world as we know it has pretty much stopped. It's not quite a Stone Age civilization reset but definitely back to the Iron Age.
From "Effects of Nuclear War" by the US Office of Technology Assessment:
Most economic damage would occur from lost production...The major damage to the economy, however, would result from deaths and long-lasting injuries (to consumers and producers), and personaI tragedies and other traumas making people less able to work. The magnitude of economic loss could be expected to vary with the number of deaths. The attack would cause considerable economic disruption in the uncontaminated area. Facilities there would need to produce a vastly different mix of goods and cope with the absence of goods that normally come from contaminated areas. Until people acted as if they believed the war was over, it could prove difficult to organize production in the uncontaminated areas. Uncertainties about the legal and financial arrangements that support production (money, contracts, credit, etc.) following a nuclear attack might impede production in the uncontaminated areas. Public disorder could also impede production. The changes and uncertainties would cause some economic disruption; however, the greater effort put forth would probably more than compensate for it.
Emphasis mine.
Basically, questions about who gets paid for what, by who, and by what mechanism would cause a drastic reduction in economic output.
Every dot on the map below is gone. How well is civilization going to work when those are all gone?
(source: citylab.com)
Oddly, in this scenario, North Korea might end up doing pretty well but only because they don't have anything to begin with.
Moving asteroids? Antimatter? Relativistic shells? Surely you jest! That's not energy efficient.
The most efficient way is to beat mankind with their own weapons.
Hack into the President's Red Telephone, or NORAD, or some silo in the Midwest and let the nukes go ballistic. Wait for counterstrike and mushroom clouds to settle. For extra effect, release viruses and spray whatever nerve gas is still in their B and C arsenals. Done.
That advanced civilization would only need to listen in to our military radio communications for a while, use their massively parallel quantum computer to break humankind's most advanced encryption algorithms (their NSA is beyond what you can imagine) and radio down the launch sequence codes. This is wonderfully efficient because the ICBM target coordinates are specifically selected to cause maximum damage or disruption by our finest military intelligence. The aliens wouldn't have to scratch their tentacles just where to point their beams of death. The same argument also applies to B and C weaponry: specifically designed to wreak maximum havoc on human physiology. Why abduct humans and study their weaknesses when they already have incorporated all that knowledge in their weapons of mass destruction^W^W^Wself-defense tactics?
Total energy consumption: a few Joules for carefully modulated electromagnetic waves.
Hit the planet with an asteroid.
Guide a 100 km wide asteroid at the planet at around 10 km/second and there will be a global extinction event. Anything larger than 3 km can cause extinctions, so the 100 km figure is simply to make sure the job gets done right.
An electromagnetic pulse (EMP).
We want to destroy civilization - not destroy the planet.
It's evident the Aliens possess some pretty interesting tech, else how would they have got here? I'm pretty sure that the energy requirements for interstellar FTL are massively high - orders of magnitude greater than anything created or generated on earth.
This means there is some advanced energy generation or storage at play.
The Alien race would be able to potentially rapidly 'discharge' this energy into the ionosphere - instantly knocking out satellites - so there goes communication and GPS.
Up the power and you have a full on Geomagnetic_storm - with all the devastation that this would cause:
Radiation sickness
Electrocutions
Complete power grid failure
Global communication outage
Devastating weather effects (Hurricanes, Typhoons)
increased tidal forces (Tsunami, Earthquakes)
All of which would be impossible to fight against, and all the aliens need to do is observe, and increase the power as required.
Post a query to the human's most popular Q&A site, then wait for the humans to list all of their vulnerabilities and easy ways to kill themselves.
Wait for long enough for the question to go quiet. Then choose the method with the most up-votes and see whether in fact your alien technology can implement it. If so, use that method. If not, continue down the list in up-vote order until you find one that you can implement.
Best part? This will work for any form of life with a large data-centric society.
The absolutely most efficient way would be to land in contested territory somewhere, and claim that you want to give some superior weapons technology to the leading nation of the world.
Then sit and watch how the nations destroy each other as they try to make sure they are the leading nation. It will help if you have defensive technologies that can shield you from nuclear blasts.
Since it is already for human grade technology a little hard to imagine bringing up all the energy needed to travel and do all the things, lets just add a little more and use
These could also be a convenient way incorporated into the travel of these species. Adding enough energy to a flipper steel ball to make it travel at significant fractions of $c$ can deliver the amount of energy of a hundreds of megaton nuclear weapon, without the drawbacks of the massive fallout (there will be some, but it is likely much less than with conventional thermonuclear weapons).
It will also be a convenient way to slow down when traveling. It is fair to assume that they travel at significant fractions of $c$ to actually get somewhere in time, so firing enough of these balls gradually at earth will slow their ship down, obliterate the earth (no need for aiming, no one will survive a steady stream of thousands of hundred megaton bombs arbitrarily scattered around the surface), and if timed correctly by the time they get into orbit, everything is ready for whatever they want to do.
Does blowing up the planet count? In that case I've always liked Greg Bear's idea in The Forge of God:
Fire two pellets of neutronium (neutron star material) at the Earth, one made from neutrons, the other from antineutrons. Due to their density they won't collide with Earth, instead they'll fall straight through it as if the Earth didn't exist, orbiting around the Earth's centre of mass. The antineutronium pellet will spark a little, but it won't annihilate catastrophically. Until they meet at the centre of the Earth, and then: kablooey.
I don't know how scientifically accurate this is, but it sounds legit. If the alien civilisation's intent is to use the Earth for their own purposes then they might consider this slightly overkill... :)
After reading interesting Aracthor's answer, I had the idea to do the opposite. Instead of hide the Sun, lets show more of it to Earth.
You would need to put a bunch of giant mirrors and/or lenses in orbit on outerspace and keep them properly aligned to reflect and focus the sunlight to Earth. This is doable with some small robotic mirror-bearing or lense-bearing starships powered by solar power.
With enough mirrors and/or lenses in orbit reflecting the Sun light direct to Earth the result would be that Earth temperatures would rise. With a reasonable area of mirrors you could get global warming, weather disruption and famines. With a much larger area, you could venusform* Earth.
* I coined that verb as something similar to "terraforming", but instead of making something like Earth, it would be like Venus.
If you find a way to completely block the sunlight to shine on a planet, you can be sure every life form should disappear quickly.
So just prevent the planet from being shined for one week, and you would destroy any vegetable or animal life form on the surface. Even if there would be some survivors that would have hide in some shelters, you can let the sun out to watch them starve to death.
Stolen shamelessly from one of my previous posts.
If you absolutely, positively need to sterilize the surface, you must blast it from orbit. It's the only way to be sure.
The problem you'll quickly run into is that the Earth is big. You can crash 10km sized asteroids into it, and there'll still be some survivors to whine about the injustice of it all. Now, we wouldn't want that, would we? There are two surefire ways.
No, the only way to be sure is to be thorough. You'll need a fair bit of antimatter, um, let's see...
1 Mt is $4.1\times10^{15} J$ so the Tsar Bomba (in the tested config) at $42MT = 1.72 \times10^{17} J$
2 kg of $E=mc^2 = 1.79 \times 10^{17} J$
So each 1 kg antimatter bomb (as it anihillates with 1kg ordinary matter) would have a blast similar to the Tsar Bomba, the most powerful nuclear device ever detonated in the puny humans' history. That gives you a nice $1,200 km^2$ blast incineration area. Now, if we were sloppy, we would just pepper the land area of $148,300,000 km^2$, so about 1,236 MIEVs (Multiple Independently Targetable Entry Vehicles) with 100 1kg (+containment) warheads each would do.
But that would miss all the boats, planes and submarines crowding their primitive buoyancy based transportation lanes. With about 100,000 ships out there, that's a lot of survivors. $361,000,000 km^2$ to cover, you'd need another 3,009 MIEVs. You might still miss a submarine or two, but without space launch capabilities, zero industrial capacity, and an all-male crew, you're set anyway.
Pro: Clean blasts, little of that nasty radioactive slag. Also, once in atmosphere, any attempt to shoot down will only damage the containment system and detonate the antimatter.
Con: You need a lotta bombs.
Thanks to our friends at Wolfram Alpha, I don't even have to do the calculation myself. Take your standard light-crafts, a 3,000 ton craft, push them to a reasonable 0.25 c and crash them on opposite hemispheres (preferably from multiple directions, but you could also do several hours apart). Each will clock in at around $8.8\times10^{21}J$, or about 10,000 times those anti-matter firecrackers from earlier, or about the same as a 10 km asteroid impact. To be really sure, send a few dozen of these.
Pro: Almost impossible to intercept once acceleration is complete.
Con: Your reptilian colonists awaiting in orbit might themselves cook a bit from the impact gamma blasts.
Meteor
Don't bother nuking major cities. Nuke space rocks onto a collision course. Use a few for a faster results! A small moon would be a great candidate! But that's pretty messy. And after all, as a stereotypical alien invader, you're looking to colonize. Are there any cleaner options?
Virus
You knew you had a reason for all those abductions and experiments. Upload a human population in your brand new quantum simulation program on your ship's super computer. Now in supertime, evolve a virus that is guaranteed to wipe out those pesky sapiens! You could start out with a particularly nasty strain of HIV. When you're ready just drop a pod into a major international city, don't want it to burn up in the atmosphere!
Self-replicating machines
You just realized that you are an advanced race, and biological warfare is a pretty low-blow. And you've been watching some DirectTV while in orbit and an old sci-fi flick with a big Austrian guy inspired a brilliant idea. Drop a single small machine loaded with instructions to mine up a bit of resources and complete, then replicate itself. Finding a location with a suitable amount of resources available your colony will be able to double itself in hours. Within 20 doublings (maybe a week or two) your army will consist of over a million robotic soldiers awaiting your command. And when they are finished with their eradication mission they can start the construction of your new palace. And to think it just cost you one 3d printing of some plans you found on the galaxy-wide-web. Just make sure they say catchy phrases when eliminating the targets, like "Hasta la vista, baby."
Terraform
All this time you've been absentmindedly setting the planet's forests and crops on fire with the ship's laser canon, just for fun, when it hits you! You've been getting ahead of yourself. The planet won't support your new family as is. Now you feel silly for entertaining all these ideas. The eradication will solve itself! You drop one Ready-Made Multi-Purpose Automatic Self-Constructing Terraforming Factory Seed 2000 on the planet. Now you sit back and relax. The planet will be a nice and cool 98 centigrade with plenty of that life giving arsenic in the water in no time.
Note: While the tone of this comment is lighthearted, the science is serious. None of these things require the alien to have huge amounts of energy or resources before hand (like relativistic projectiles or antimatter/neutronium bombs). In addition it accounts for multiple different end scenarios (such as destroying the everything, or just the biology, or just one species). It also leaves room for multiple starting situations like a lack of suitable asteroids/comets, highly exploited or otherwise unavailable resources. It's impossible to say what the most efficient method would be for an unknown alien on an unknown planet wiping out an unknown civilization, but we can make some good guesses I think.
How About I don't attack earth directly, but rather instigate the sun to fire a massive 24 hour long barrage of solar flares in earths direction. If the flares contain a massive amount of radiation they will kill almost all life by radiation poisoning, burn the face of the earth, and the electromagnetic disturbance will destroy most orbital and local electronics. After 24 hours the whole surface of earth was bathed in the storm and civilization will be wiped out.
And they lived happily ever after.
Make a survey of the planetary surface and collect samples of the most potent types of virus affecting humans. Use your sophisticated biotechnology to engineer them to be more transmissible and more lethal. Release them into major urban areas.
That will require no further action from you and will result in sufficient loss of life that civilisation will break down. There will be humans with sufficiently potent immune systems to survive, but very few of them and they will now be living in a perilous post-apocalyptic world. Your mothership landing will be the least of their worries.
Depends on which definition of "efficient" you're looking at.
Efficient: achieving maximum productivity with minimum wasted effort or expense.
If you're really shooting for minimum wasted effort or expense, couldn't you just do nothing and wait around? On a long enough timeline most all civilizations are ultimately doomed; the sun will super nova or burn out eventually, large asteroid strikes happen every so often, and so on...
Criteria: Either cheapest or least energy consumption.
Patience doesn't cost much and requires very little, if any, energy.
Consume all the oxygen. Send down microbes, or chemicals that will react with oxygen, and let them absorb the atmosphere down to under 5% oxygen. There are a lot of consequences due to lack of oxygen well beyond being unable to breathe without aid. For bonus points, getting rid of CO2 would help significantly.
It's a slow process, but it's very, very efficient in terms of cost.
The solution would be to bombard Earth with gamma-rays. Normally a supernova in a nearby star would be able to do that, but since the aliens are much more near Earth and can focus their energy much better and in the most suitable part of the eletromagnetic spectrum, they won't need something as powerful as a supernova. Just a bunch of powerful gamma-ray lasers will do the work.
What would happen exactly? Accordingly to wikipedia:
The major Ordovician–Silurian extinction events of 450 million years ago may have been caused by a GRB. The late Ordovician species of trilobite that spent some of its life in the plankton layer near the ocean surface was much harder hit than deep-water dwellers, which tended to stay put within quite restricted areas. Usually it is the more widely spread species that fare better in extinction, and hence this unusual pattern could be explained by a GRB, which would probably devastate creatures living on land and near the ocean surface, but leave deep-sea creatures relatively unharmed.
[...] Gamma rays would not penetrate Earth's atmosphere to impact the surface directly, but they would chemically damage the stratosphere.
For example, if WR 104, at a distance of 8,000 light-years, were to hit Earth with a burst of 10 seconds duration, its gamma rays could deplete about 25 percent of the world's ozone layer. This would result in mass extinction, food chain depletion, and starvation. The side of Earth facing the GRB would receive potentially lethal radiation exposure, which can cause radiation sickness in the short term, and, in the long term, results in serious impacts to life due to ozone layer depletion.
Longer-term, gamma ray energy may cause chemical reactions involving oxygen and nitrogen molecules which may create nitrogen oxide then nitrogen dioxide gas, causing photochemical smog. The GRB may produce enough of the gas to cover the sky and darken it. Gas would prevent sunlight from reaching Earth's surface, producing a "cosmic winter" effect – a similar situation to an impact winter, but not caused by an impact. GRB-produced gas could also even further deplete the ozone layer.
I think to an extent, the only way to destroy civilization would be to make humans go extinct; people gravitate towards systems of government, even if only at a local level. At least one system would probably grow over time, so the only way to make sure it's completely gone would be to kill everyone. While you could do that with an asteroid, it would be extremely hard to actually change the trajectory of something that large, unless you're willing to do the calculations, tap it slightly and very precisely while it's far away (100's of years) from earth, and then just wait around for it to hit. A much faster, easier method would probably be to just nuke the place. If you don't have enough to hit everywhere, then probably just hitting the major centers of government, commerce, woods/trees, and agriculture, would be sufficient to end civilization in fairly short order. Just make sure that you destroy enough so that any survivors won't be able to find food, fresh water, and/or shelter, or maybe even heat or oxygen if you can set off enough firestorms and destroy enough forests. Clouds of smoke covering the whole world would deprive it of sunlight and heat, which in turn would greatly facilitate the cold death of the planet. Ain't it depressing that we're capable of doing all that right now.
Why destroy when you can enslave? Destroy 1 major city, televise it, jam all frequency, and say "Join us or you're next"
If we assume the aliens are travelling at relativistic velocity between the stars (no FTL), then they have several options:
Launch a probe at a high fraction of c at the Sun while beginning your braking manoeuvre. The impact on the Sun will trigger massive solar flares and other energy releasing events which will destroy virtually any space based technology that we currently have, and fry much of the electrical and electronic infrastructure of the Earth. The world's economy will crash, and in most of the advanced nations, where farming is highly mechanized, the number of farmers left will not be able to feed the local population (much less the global population), and mass starvation will take care of the bulk of the world's people. If diverting part of the US corn crop to make ethanol cold raise food prices to the point of causing riots in Mexico and other nations around the world due to price increases, imagine the effect of stopping ALL food exports.
If they are moving at a very high fraction of c, they might just "buzz" the planet. The Lorentz Transformation of a sufficiently massive spacecraft would be like moving a massive object through the solar system, potentially disrupting planetary orbits. The Earth could suffer disruptions ranging from massive earthquakes to being moved bodily from the habitable zone, depending on how the aliens times their pass and how "massive" their ship appears in our frame of reference. (If the movement brings Earth into "their" idea of the habitable zone, so much the better). In any event, the amount of inertia of the Earth will require a very huge amount of energy to disrupt, and this energy will be felt in the movement of crustal plates, oceans sloshing from their basins and massive magma chambers emptying.
Kill the Earth. If the Aliens don't particularly care about the Earth, they could just send a probe at a high fraction of c to strike the planet directly. At .99*c*, even a relatively small object will crack open the crust and expose the mantle (http://what-if.xkcd.com/20/). It might actually be more "efficient" to send a shower of objects timed to strike all areas of the Earth as it turns in its orbit over a 24hr period to sterilize the planet without cracking open the crust, but even the energy release at that point will probably boil the oceans. One could imagine a shower of marbles being dumped from a missile bus moving at .99*c* as the impactor, at that speed, the material hardly matters.
One of the advantages for the aliens is that it will be virtually impossible for Earth to see the incoming projectile (regardless of the target) at high fractions of c, it is coming virtually behind its own light, if you do happen to see it, it will be "in the past", and you won't have much of an idea where it is now. Even with futuristic technologies, there does not seem to be any way to see or stop highly relativistic projectiles coming from space.
We, humanity, already have the technology to destroy ourselves. Why complicate it with antimatter or near-c moving objects? Ever seen The Day the Earth Stood Still?
I see two options that are both cheap. One is relatively quick, the other is kind of slow:
Everything will die in about a month. Cheap, easy, and relatively quick.
How?
Nuclear detonation in the atmosphere in key locations should do the trick. Someone already suggested hacking into the already existing systems on Earth for launching nuclear weapons. That sounds like it would cost only a small team of nerds and a ton of red bull. Alternatively, Uranium and other radioactive elements are pretty easy to find in the cosmos, I'm sure. Mining operations are probably not even marginal in operations management for this alien race.
Problems
The planet's irradiated now. Unless you have some way to reverse it you won't be able to use it for a very long time. All surface life is dead, not just humans.
Pros
The infrastructure remains. Everything you ever wanted to know about humans is now enshrined in the remarkably undamaged ruins of its cities.
This was already suggested, but a virus strain that specifically targets humans and spreads and kills quickly would work very well.
How?
A combination of illnesses, covering multiple contagion pathways, all released in multiple locations around the Earth.
Problems
Not as cheap as irradiation, and not as efficient. You would need time for research, probably years, to determine and develop the most effective pathogens. There would also be survivors, whether immune or unexposed, that you would have to dispatch manually.
Pros
This leaves the planet human-free and relatively unharmed. If you do it right, you will get well over 90% of all humanity and nearly 100% of those in the cities.
All others get pretty excessive (i.e. wasteful).
A pretty efficient way to destroy the earth would be to use a high enough powered laser from space to melt through the Earth's crust at its weakest areas (such as supervolcano caldera) to cause a volcanic winter.
The idea is similar to the other ideas of causing extinction mentioned here, such as nuclear winter and just outright blocking the sun, but i think this has the benefit of really requiring very little extra technology beyond our current capabilities. All it needs is a big enough laser, powerful enough to melt silicon and whatever else is in the crust, with big enough focusing lenses of a material that can withstand the intense energy.
Outcome: These statistics are simulated projections for nuclear winter but i think you could draw a pretty direct comparison. Not the fastest way to do it, but i think probably the cheapest and most energy efficient, as well as being technologically realistic.
Negate or Warp the Earth's Magnetic Field
Either temporarily negating, or warping the magnetic field of the planet such that the sun facing side effective has little to no electromagnetic protection. The solar wind would wreak havoc on anything electrical or electronic and on anything on the surface containing DNA (i.e. people, plants, life in general.)
Negation could be effected via phase cancellation, altering the shape of the field can be effected via the employ of extremely strong and large directable fields. Electromagnetic field manipulation on this scale would be especially feasible by a sufficiently advanced alien race capable of interstellar travel. I do not see anything that would preclude such a possibility as such massive fields would likely be used in shielding, containment, the actual method of interstellar travel via the manipulation of the actual physical "shape" of space surrounding the craft, possibly directed energy inter-vessel weaponry, and so on. They would certainly be capable of harvesting, harnessing and/or producing the requisite energy to do these things. It might even be likely that such a vessel would be powered by some kind of a contained/managed stellar, neutron star, or even a black hole based "power plant" or "engine."
Perhaps even a complete and permanent collapse of the planet's electromagnetic field could be caused by focusing complex, directional and huge fields on the molten core of the planet thereby causing a cessation of its rotation therein!
A few days of this would be sufficient to wipe out any real civilization, kill a large portion of life on the planet, trigger lots of mutations among the remaining life and make earth an all around fun place for the few politicians who hid themselves in bunkers--if they had the time/wherewithal to do so before being affected beyond survivability--as they emerge thinking they'll have something left to run...
Not a joke: jam their smart phones.
Not as sexy as that antimatter device, but sadly much more effective and energy saving. Who said aliens civilizations wouldn't care for green conquer? That's what Douglas Adams would have thought about :-)
I find it unbelievable that no-one has mentioned the single most efficient weapon for destroying civilization, at very low energy and economic cost.
https://en.wikipedia.org/wiki/Biological_warfare
a couple of quotes from the above wikipedia:
Offensive biological warfare, including mass production, stockpiling and use of biological weapons, was outlawed by the 1972 Biological Weapons Convention (BWC). The rationale behind this treaty, which has been ratified or acceded to by 170 countries as of April 2013, is to prevent a biological attack which could conceivably result in large numbers of civilian casualties and cause severe disruption to economic and societal infrastructure. Many countries, including signatories of the BWC, currently pursue research into the defense or protection against BW, which is not prohibited by the BWC.
That is, we are talking about a weapon less legal than nukes. And here's why.
It has been argued that rational people would never use biological weapons offensively. The argument is that biological weapons cannot be controlled: the weapon could backfire and harm the army on the offensive, perhaps having even worse effects than on the target. An agent like smallpox or other airborne viruses would almost certainly spread worldwide and ultimately infect the user's home country. However, this argument does not necessarily apply to bacteria...
Aliens would have an entirely different physiology and would almost certainly be immune to human viruses, which have specifically evolved to reproduce in a human cell. (On the other hand, simple bacteria and parasites might prove deadly to the aliens.)
So if theres a wave of alien abductions, they're probably taking people to use a guinea pigs to breed and incubate the ultimate strain of smallpox. Although the disease has currently been eradicated, we shouldn't get complacent and stop vaccinating, because when aliens attack we're doomed.
https://en.wikipedia.org/wiki/Smallpox
https://en.wikipedia.org/wiki/Smallpox_vaccine
Note that smallpox played a major role in the conquest of the Americas. The Europeans brought smallpox with them, and the natives, having had no contact with the disease, were decimated. (In return, it is believed the Native Americans probably gave the Europeans syphilis.)
Aliens should research our culture and gain knowledge of the planet, before they will destroy us. I believe from perspective of inhuman specie I would've destroy human kind with the most creative way as possible. If we can convince entire human race that after you die, you become into anything you wished. Christians will go to heaven and atheists will become gods of galaxies, muslims will travel to gardens of virgins and etc. Maybe we the aliens will pour some toxic delusional drug into the atmosphere. Make everyone trip into dreamworld of endless possibilities. Countless casualties by suicides. The ones that keep breathing on this planet only because of fear to die. Yeah orbital strike with enhanced LSD is my final answer.
Destroy the 3rd world countries and wait for civilization to collapse. Modern first-world countries depend on the third world countries for so many resources it would be difficult to function without them. After all, those countries supply large amounts of the resources necessary to build a F-16 or any other modern weapon. The destruction of the 3rd world countries could be carried out via a bioweapon or something else. This has the added benefit of minimizing damage to the planet.
Throw a tether from the Moon.
OK:said like this, it need almost infinite strength. But if you make something that hung until the earth atmosphere, the aerodynamic drag will slowly slow then crash the Moon down.
This still need a science-fictionish tensile strength, but the energetic cost can be very small.
A lot of people have been ignoring the energy-efficient aspect of the request. There's a lot of things that would work but use a huge amount of energy.
The most efficient way to destroy civilization would be from the Kupier belt. Take something say 20 miles across and deorbit it. Gravity does most of the work. It's slow, though.
They could use their "magic-plot-teleporty device" to teleport a large chunk of the nearby star's core into the planet. That'll do it.
