When the ponies came, they tried to convert humanity with words and kindness, saying that they were misguided beings and the princesses would help them on the path to Harmony. When that failed, the ponies tried the potion, only for it's magic to be countered by humanity's science. Then they tried force and failed completely at that.
Finally they decided to let the Barrier defeat humanity and it seemed to work! Nearly 50% of the planet was covered with humanity doing it's best to find a solution.
Unfortunately, the ponies of Equestria are about to learn a harsh lesson that humanity learnt long ago. The Earth can be paradise for those who tame it, but to others it is hell.

The Conversion Bureau never made any sense anyway, but here are some places where it would have made less sense.
Not necessarily worlds where the ponies would lose the fight - just worlds where conversion wouldn't logically work.

Before suggesting a World, read the index to make sure I haven't done it already.

Humanity is dead.
It's members have been converted and brainwashed. Its homeworld has been absorbed, and its culture has been eradicated.
Princess Celestia couldnt' be happier.

Unfortunately for Celestia, there are beings out there who don't look too kindly on xenocide. Beings much more powerful than her...

Inspired by Rush's "The Conversion Bureau: Celestia's Silly Plan"

Author's note: For the record, I don't hate all TCB fics. My problem is with the ones that depict humans as irredeemable bastards whose only hope for salvation is to convert themselves into talking horses. They piss me off. They really piss me off.

After decades of brutal warfare, humanity has finally breached the defenses of Equestria, and now strikes at Canterlot itself. High above the walls of the city, Celestia watches the world she built burn around her. Her gambit has failed. Humanity has endured her assault. Now all she can do is wait and watch as the war she started comes to an end.

Celestia, attempted conqueror of Earth stands atop a roof with nothing but herself for company. One year ago she failed to bring harmony to earth by way of the the Conversion Bureaus, and humanity has begun to reclaim what it lost. She has been punished for her genocide, though she wonders if her sentence was too lenient.

Written as a speed-writing exercise. Completed in about an hour, and it's unedited. It's just a concept I've had for a while and wanted to see what others thought of it.