Inspired by a number of things. Chiefly among them, my shrinking sanity, delusions of being a halfway decent writer, Wasteland Soul by Miracle of Sound, and Khat. Fallout 3 crossover with Fallout: Equestria. I heavily recomend enjoy both entertainments before reading this.

Mirrors have been used as portals for a very basic reason. Magically speaking, symbolism is very important. Mirrors imply a reflection, a certain duality. For everything on mirror side, will have a corresponding piece on the other side. To mirror the magical achievements of Equestria, Humanity developed sciences.
The war with Zebrica pushed Equestrian magics to exhaustion. Only unicorns could develop, and use most magics. Applejack would not stand for leaving the other ponies behind. She authorized more invasive studies. Equestria could not just rely upon magic anymore. They needed technology.
What they found on the other side, was breath taking. A familiar world filled with marvels of technology. This gave the Ministry of Technology a massive boon. They began producing something thought impossible to build, even with magic. Powered Personal Combat Armor.
Corruption started ruing rampant in both worlds, as leaders became further detached from the common folk. Propaganda speakers floating across both the nations. Economies wrecking themselves to fuel the war machine of their nations' army.
Then, it all came to a head. It all went wrong. Balifire rained across Equestria. The Pink cloud engulfed the center of culture. Across the mirror, a nuclear explosion triggered responses from China, which triggered America's own nuclear arsenal.