Equestria. A land where once three tribes of different ponies lived in harmony: the Earth ponies took care of the agriculture, producing food for the tribes; the Pegasi controlled the weather, and the Unicorns used their magic to bring the day and the night. Though they lived separated from each other, the three tribes had a good relationship between them. But the selfishness of one of them, led to a disagreement.
The Unicorns discovered enormous amounts of precious metals in the undergrounds of Ponyville, the most important town of the Earth pony tribe. They insisted that the Earth ponies moved to another place to permit access to the richness. But the Earth ponies refused to abandon their beloved home, which caused the anger of the Unicorns.
Moved by the desire of reaching the metals, the Unicorns threatened to declare war against Ponyville, in case the Earth ponies refused to move once again. The Pegasi tribe interfered, warning the Unicorns that their tribe would fight beside the Earth ponies in case there was a war. This declaration made the Unicorns tribe fearful, for the Pegasi were known for being great warriors.
And there was more than that: the three tribes had similar numbers of population. If both the Earth ponies and the Pegasi fought against the Unicorns, the Unicorns tribe would be out of numbers, and the chances of victory would be slim. But they weren't about to give up.
The Unicorns had in their power a magic mirror, that was actually, a portal to another world. Nopony has ever crossed, but the Unicorns' commander, Sunset Shimmer, saw in the portal a chance to find more warriors to fight by their side.
After crossing the portal, proving the existence of an alternative world and explaining the situation, the Unicorns proposed to the inhabitants of this other world, the humans, to join forces with them. In exchange, they'd share the metals with them and permit access to explore their world, learn about it and even live there if they wanted.
Lots of people agreed, some moved by the love for adventure, others for the chance of studying a new world, and most of them, for the richness promised.
It didn't take long for both the humans and the Unicorns to notice that each one of them had a counterpart on the other world. At first, it was hard to deal with. But there was a point where the Unicorns commander decided to use it as a benefit.
A Pegasus had died, a hero respected for the whole Pegasi tribe. Maybe if the Unicorns found his human counterpart, they could send him to win the trust of the Pegasi and convince them to abandon the war, making much easier for them to have victory.
And they did find him.
