Maybe some alien civilization does know about us, but it slightly afraid since we spec’ed in the war tree instead of space travel.
They kept letting us advance our war machines until one day they needed us. To help stop a greater evil; since they went space exploration instead of guns
Except the moment you have orbital dominance over a planet then no amount of ground or atmosphere based weaponry matters whatsoever. All they would have to do is drop some mass from orbit and they can cause destruction far beyond even the most powerful nuclear weapons.
What you forget is that weapons are just an application of the sciences. Superior science = superior weaponry, even if the aliens haven’t bothered to “spec into” weapons research.
Just steer a huge asterioid into the planet.
Footfall nails this. Aliens show up and drop rocks on us from orbit. The reason we prevail is the only plot I’ve read that makes sense.
How To Beat Aliens
They didn’t invent the ships or tech they’re riding in and aren’t much, if at all, more advanced than us. Plus, we figure out their rigid social hierarchy and turn it on them. Also, we nuked Kansas.