Everypony has a fear of the end of the world. The fear of losing everything.
The fear of no longer being able to live your life.
But that wasn't a fear brewing in anycreature's minds. Why would you worry about civilization being killed off when there was no imminent threat? Of course, if there was such a thing, the princesses would protect us. Celestia and Luna be blessed with our love!
But sometimes, there comes a period where nature turns against us, and the reasons are unknown. Nature is a crazy thing isn't it? One day, it gives you a healthy apple to eat, another day it turns your world into a living hell. And when the latter occurs, it's hard to remember the former. Negativity always seems to outweigh positivity, doesn't it?
And that's exactly what happened to the planet Equestria sat on.
With advancements of technology in the kingdom, Equestria had become able to see past the atmosphere. Reach into the stars, as they said. A vast void filled with other stars and worlds seen with a clarity never thought possible. Space travel wasn't a reality, but nonetheless, ponies were amazed by what magic and science could achieve. It caught the attention of outside nations - with many races coming to Equestria on their own accord to get closer looks, and learn from it.
It brought further harmony. Technological feats were achieved abroad.
Most of that, however, had come to an abrupt halt. One fateful afternoon, a group of Pegasus researchers in Cloudsdale had detected a sort of flying rock - a meteor. At first, nobody thought much of it - they had discovered other bodies such as this one, and there was a near-zero probability of it striking Equus.
Near-zero doesn't mean zero though.
The meteor, codenamed Pegasus-04 for obvious reasons, grew much more alarming as years passed. In short, it wasn't just some random miles-wide rock in space. It was a miles-wide rock in space on a crash course towards Equus.
The princesses were informed, and soon, everycreature knew. Equestria devolved into panic - even the Mane 6, the Elements of Harmony themselves, were worried. Other countries, other species just as deep in trepidation, had sought to survive; some went high into the clouds, others went deep underground, but they knew it'd be a lost cause. Even during attempted international negotiations, it was always known nothing could withstand the impact of a meteor.
As Pegasus-04 edged closer towards the planet, an idea was hatched: What if a shield was put up to protect from the impact? Of course, this would be thought as stupid, but now that Equus was going to be turned to a wasteland, no-one cared about what was 'possible' and 'impossible'. The instinct to survive was in full swing, ponies, griffons, dragons, etc. alike.
Amazingly, through some sort of scientific miracle, the shield was swiftly devised and established - 6 generator facilities placed around the outskirts of the kingdom, with a central HQ in Canterlot. The princesses and Twilight ran the HQ, the others by the rest of the Mane 6. The activation of the barrier commenced just days after the final facility was put in place. Ponies were not the only races to create such a thing: all the other creatures offered to aid development in every way applicable.
Unfortunately, due to techno-magical limitations at the time, the shield had only protected Equestria. After several failed attempts to expand, the princesses traveled to other areas - Griffonstone, Seaquestria, Yakyakistan, even the Dragons Lair. They wished to try and protect as many as they could, Equestria acting as a sanctuary. They promised that as soon as they were able to, they'd bring the shield over to, and rebuild, their homes. The sisters also affirmed they'd help further educate them on the shields and generators, hoping they could utilize them themselves one day.
When the meteor pounded into the surface of the Earth, it brought forth an inexplicable amount of fire and ash, blocking out all sunlight and trapping the planet in a hellish environment. Nearly all life had been wiped out, and the crater of the meteor had become a massive magma pool. It seemed, truly, like the end of the world.
But Equestria survived. The shield worked.
Though not completely unaffected, society had turned into a new chapter. The world outside had stayed uninhabitable over the decades, but everycreature persevered. Slowly, towns expanded, cities evolved, new jobs were created, and the economy grew.
The shield stayed impenetrable, forever safeguarding the kingdom, and every single organism inside it.