Just a thread to help the human race survive an alien invasion. List obscure alien warfare that most wouldn't consider. No giant mother ships or death stars. I'll start. Releasing an invasive or disease carrying foreign species on our planet. (Alien mosquitoes that give out super AIDs)
Using their ships to spread somekind of a cloud around the earth to hide the sun, and thus essentially breaking our natural habitat?
Well, we have Biological warfare, Ecological warfare, I guess I'll add to the second. Perhaps redirecting an asteroid to impact the planet? How about crashing some sort of balloon, or ship(s) that contains a crap-ton of CO2, possibly causing a runaway greenhouse effect (that one though is much longer time-wise).
Confederate leaders. Basically, the Alien's would implant/infiltrate/mind control young children from all over the world and redirect their life courses. They would guide those children through their lives carefully, using their advanced alien knowledge of human history and behavioral trends to cultivate the children's leadership potential and likability. The children would be natural choices for political careers, with each one eventually being selected to lead their host country. They would then guide the children to lead their respective nations into a series of complicated, and evermore destructive conflicts, resulting in the Earth remaining in an perpetual state of carefully managed war, with our destruction never certain, but always possible. ... Oh, wait.
Aliens could overtake the earth with their superior skills in sex, and create a massive amount of hybrid. Resulting in a slow takeover through the power of sex and not of violence. Now that is a conquering with a happy ending.
Infiltrate the economy, build giant factories to spew CO2 into the atmosphere, wait til the resulting climate catastrophe kills off the humans and makes the planet habitable for your species. And if any humans get wise to your plan, simply infiltrate the news media as well and say stuff like "It's not CO2 causing global warming, it's sunspots!" That and a few well-placed bribes will keep the authorities off your back for as long as you need. Oh wait, that one's already been done: https://en.wikipedia.org/wiki/The_Arrival_%28film%29#Plot
What, the sock monkeys? They lived under the bed for so long they grew into a whole new organism.... that's why they're called sock monkeys.
How about an oxygen bomb. Deafen 100% of the world by raising the oxygen content in the air without killing anyone. Directly at least.
I remember reading some short science fiction stories. I think it was a collection of Isaac Asimov, but maybe not. The basics were: There are martians, and the human race is trying to co-exist with them. The martians find Oxygen toxic, so use Oxygen in their weaponry. The humans send androids to negotiate a peace with them, and the androids wind up scaring the martians. The martians are advanced enough to manufacture technology in industry and all the rest, but have never met a human before. The androids are attacked with oxygen weapons by the martian military when they establish first contact, which does nothing to them. They then explain they are not hostile and would like to explore diplomacy. The martians try to intimidate the androids with their industrial and political prowess. They tour them around their refineries and manufacturing sites "Look how glorious our industry is!"etc. The androids scare them, because they do things like observe the inside of a foundry without any heat protection, by wading through molten metals and taking samples. The story ends with the martians being terrified of the androids and being very cowed towards them as opposed to their initial reactions of "we are better and stronger". In the end, they apologise for their behaviour and say something along the lines of 'I had no idea you humans were so strong, please don't destroy us' thinking that the androids were human beings. The androids replied 'We're not human. We just work for them.' which scared them even more. I don't know why this popped into my head, but I thought it worth mentioning as an 'out of the box' short story. No other reason. Also, to point out, they weren't human shaped androids, they were tank tread/armoured machines. The martians don't know what humans look like.
The Marvellous Galactic Empire relocates Species X to this little blue and green planet after it becomes endangered. Species X quickly reproduces there and winds up destroying human civilisation as we know it. The story is told like a nature documentary to the rest of the universe as no difference is seen between human civilisation and Species X on a technological standpoint. The eradication of the human species on planet Earth isn't viewed as significant as they already populate other worlds. Brian Blessed is the narrator/commentator. This is not optional.
Iscaac Asimov was a true genius. He was also kind of an arrogant prick, because he sure knew it, but hey, he was right!
Land a small wormhole creator on earth... say on the greenland ice sheet. Burrow 2km into the ice and create a wormhole to the sun... Just below the corona. The heat would begin melting the ice sheet creating a wave of superwarm humid air that would sweep the earth with torrential rain. The excess heat from the wormhole would warm the atmosphere a hell of a lot over several years making earth uninhabitably warm. Sea levels would rise. Crucially, the far end of the wormhole would be anchored just inside the sun's corona so that the pressure doesn't overwhelm the earths atmosphere, merely making t uninhabitable to us, not radioactive for others.