Luna's Banishment

"One thousand years of regret."
At the beginning of LB and at the end of the 7-year civil war between Nightmare Moon's and Celestia's armies, Celestia harnessed the power of the elements after a major battle enabled her to capture the last one she needed to banish Luna, and every single one of her followers, both innocent and guilty, to the moon. The power of the elements gave the moon an atmosphere and laws of gravity that perfectly mimic Earth's while maintaining the balance between Earth and its satellite.

Because Celestia had no choice but to banish all of NM's followers, she felt obligated to send along several thousand tons of mining equipment and seeds, trusting the unicorns and earth ponies there would be able to make the land cultivable. Since there was virtually no threat of attack for one thousand years, Celestia felt they should be allowed to build a civilization in exile.

In one thousand years, the only pony that would be able to escape would be Nightmare Moon.

After Twilight freed Luna from the control of Nightmare Moon, Equestria began the lengthy, painstakingly difficult and fragile process of bringing the ponies from the moon - "Moon Ponies" - back to Earth.

Luna barely managed prevented another civil war that would have been caused by where loyalties lie; many of the moon ponies were still loyal to Nightmare Moon. The moon ponies now live in harmony with Celestia and her subjects, as one civilization.