We detected them when the asteroid belt started dissolving. The asteroids were a lost cause: our technology allowed us to send probes that far for exploration, but defense? That was beyond us. Within the moon's orbit, though, we laid down the line. One of their ships was detected in powered orbit around our geosynchronous satellites, and a US government probe zapped it with X-rays. After a big flash our spySats could see the charred hulk remaining.
"Aliens!" we announced. "Earth is Sovereign Territory! Trespass and you will be incinerated! We see your ships, and we are coming to destroy them!"
"Right-o!" came the immediate response. "How about a toe to toe, out at twice the geosynchronous orbit? Tomorrow 3:30pm Greenwich, perhaps?"
The earth people consulted. Available rocketry, orbital trajectories.
"Um ... tomorrow's not good for us. How about ... three months from now, May 25, noon?"
"Jolly good! We'll pencil that in. Showdown at High Noon! Um, would you like help with any of the heavy lifting?" As the message arrived, alien ships could be seen coalescing out at the specified distance, spreading out solar cells, shifting back and forth for position.
"No no we've got things in hand."
Back in the 60s earth had designed an adequate response. Project Orion. A city-sized spaceshift that would be propelled by atom bombs. Certain secret desert spots became a beehive of military activity.
The aliens, in stealth, had all nations, cities, even streetcorners riddled with nanotech monitoring, recording everything. Project Orion, secret military tests, secret political finaglings, leaders snoring in bed, everything. Occasionally they would mock up a little green man with tentacles growing out of its face to walk down main street, scare pedestrians. Often they'd have it wear a cowboy hat. Eventually it would attract vigilantes who would shoot it to pieces. The bug-eyed creatures typically exploded in a spray of green goop once they'd been shot up enough. The aliens found this pure video gold.
It took time and effort, but Project Orion took shape. The fateful day arrived. Project Orion thudded out of the atmosphere on the hellfire of repeated atomic blasts. Up, up, out towards that line in the sky. Twice out from geosynchronous orbit.
The aliens, mllions, billions, shifted about at hundreds of Gs, the size of toasters, mirrored surfaces, preparing. While they'd waited they'd dismantled all the asteroid belt and were working on Mars and the outer planets.
Orion came. It thudded up, onward, ever accelerating.
The aliens put some bulbous craft in the intersect trajectory, waiting. "That Orion thing is full of superheavy metals," observed alien commander Vufftz. "What, they're feeding us?" asked Ublix. "No, I think they intend to get them under our noses, then cause a fission reaction!" "Oh that's beautiful!" replied Ublix.
Orion came.
"Can we start shooting yet?" asked the aliens.
The earth ship, cleverly, responded by shooting first. Projectile weapons sliced through the alien fleet.
The toaster-sized aliens nimbly stepped to the side, leaving the bulbous crafts in collision course. Earth projectile weapons hit the bulbous craft, initiating a huge explosion. Reminiscent of the Death Star, actually. Earth cheered. The aliens cheered too.
Orion thudded forward. More bulbous alien ships crowded into to the intersecting path. Orion reversed thrust, but it was a big thing, it would continue through.
Escape vessels shot out from Orion. The bulk continued through, colliding with the bulbous vessels, producing another huge explosion. Orion was fatally injured, drifting on with minimal control. Somehow Orion shattered into millions of pieces, like sand, and was quickly dissolved by the surrounding alien fleet.
The aliens swarmed around an escape vessel. "You will shoot more, yes?" asked the aliens.
"Sorry, no, we're having technical difficulties," replied the earth escape vessel.
"Fight! Fight now!"
"No ... sorry ... having trouble holding atmosphere."
"We'll patch it for you! Then fight!"
"And ... we have to go to the bathroom, the toilet isn't working ..."
This prompted an extended radio silence. The aliens had all collapsed in laughter. Earthmen were Too Cute.
"OK, I think we can fix that for you," they responded, when they had recovered themselves. "And escort you back home. And, oh by the way you chaps won, we won't conquer Earth. If you'd be OK with us establishing an embassy and sending some tourists by every now and then. Is that OK?" asked the aliens.
"Um, yes? Those terms seem acceptable? But please help us now?"
The aliens escorted the humans back to Earth.
This was in response to a prompt on reddit.com r/WritingPrompts, "Humans always expected aliens to be overwhelmingly powerful and intelligent. They were right".