Arthur C. Clark once said “Two possibilities exist: Either we are alone in the universe or we are not. Both are equally terrifying.” Science fiction is rife with alien invasion plots and attacks on earth by hostile forces, from books such as “War of the Worlds”, to movies, to videogames such as XCOM and Stellaris.
Many of these things that are present in this media could actually be used if Earth is ever attacked by hostile forces, and if push comes to shove we might need every advantage humanity can get. The best part is, every single nerd in the world will be stepping up to the defense.
The biggest benefit that humanity would have against any outside threat is our unification in the face of a common threat. Games such as Stellaris have the Earth united under one government and pooling their resources together to get into space, while Xcom has a special program that sees representatives from the entire world teaming up and working as one.
While the chaos of an alien invasion could end up dividing humanity with some seeking to use the aliens for their own gain, standing together as one species against the threat will also happen. And that united spirit might even continue on after the world has weathered the storm.
Especially if the aliens are technologically superior to our earth forces, going to ground and launching ambushes and lightening raids will probably be the best way to slow down the alien forces. Pitched battles will more than likely be rare and desperate if they even happen at all.
Studying the alien threats and working behind the scenes to stop them will be more beneficial than battles that get nothing done. By hiding out and picking battles, hopefully, humanity will be able to assault the aliens and counter their advantages with surprise.
However, coordination might be a problem, and humans might need to come up with some form of communication that doesn’t rely on the internet, as any invading army will seek to cripple or control methods of communication.
By using the aliens own technology, either salvaged from battlefields or otherwise stolen or taken, and incorporating it into our own weapons and other materials, humankind will get the upgrades it needs; especially if our own weapons are ineffective against them.
It might be easy to control the tech and turn it to our advantage or it might be difficult, but it will have to be done to give the advantage back to Earth.
Of course, there are countless other tropes and strategies that are common inscience fiction and while some are impossible, others are possible and even probable to achieve. While any fight against an unknown foe is going to be an uphill battle no matter what, the battle might be a little easier to win.
We just need to stand as one, hit hard and fast, and learn to adapt. Humankind can do that. The question is, will it?