…and leaving nothing behind as proof.

If an alien commander was tasked with wiping out the human race to clear room on Earth, how would he/she/it do it? What would be the mechanism used for accomplishing this grotesque task, and how effective would it be? Would it point to whoever did it, or would it not leave a single thing behind? Incriminating evidence would not be a good thing to leave behind (some of those silly citizens might get mad), so choices would obviously be limited.

3. WMD War

Classic doomsday scenario. Grab a nuke from Russia, Iran, North Korea, or the US, shoot it off, and just wait for the string of nuclear retaliations. The great thing about this method is that it’ll clean up for you. Whoever survives the original war will die off from the radiation that the weapons would release. There’s also the extra benefit that there won’t be any buildings left on Earth, and thus no trace of the previous, puny civilization that inhabited the planet.

The drawback to this method would be the fallout that might kill off some people of your own race when they come to inhabit the planet. Or maybe someone would find charred remains of buildings located somewhere on the planet. Still, if speed is what you’re looking for, no method beats this.

Although if Earth is completely devoid of all life then the initial incentive for settling would be gone… maybe a minor problem.

2. Nothing at All

Step one, wait. Step two, profit.

I’m sure that global warming or some other natural catastrophe will wipe out the human race eventually. Hell, there are scientists predicting that a huge asteroid will hit the earth in 2012 and wipe out every trace of life. If nature doesn’t cause the end of humans, then I’m sure they’ll find some other way to screw themselves over.

Not the best method when it comes to timeframe, but if you’re the conscientious type, this method is the best.

1. Mass Abduction

Suck up all the people on Earth, and turn them into slaves to power your big, powerful machines. You’ll have to do the dirty work like erasing all traces of architecture, but this is probably the most efficient method anyways.

I’m sure that if your citizens found out what you were doing then there would be some ethical objections, but then you can enslave them too. Hey, you didn’t travel millions of lightyears for nothing!