Clyde and the Emperor's New Clothes
Long ago, an eminently sensible earth pony named Clyde (a distant ancestor of the one we know today) lived in the northern empire under the iron hoof of the vain and greedy Sombra. Despite the harsh living conditions and the terrifying crackdowns of the secret police, many were the ponies who tried to curry favor with Sombra — since he faced the constant threat of rebellion, and those who helped him solidify his rule found themselves lavishly rewarded indeed.
One day, Sombra decided that he needed new formalwear befitting his grandeur. While suitmakers across the land tried to outdo each other with ever more ostentatious designs draped in jewels and exotic fabrics, two very clever unicorns showed up in his throne room with nothing but a large empty box and some tailor's tools.
"I have special dungeons for those who waste my valuable time," Sombra said, charging darkness into his horn.
"Wait! You'll like this," the clever unicorns said. "We have gone throughout the land to collect fragments of the finest, most perfect shadows, and woven them into a miraculous fabric lighter than air, softer than silk, and stronger than steel." They moved their hooves as if unfolding a suit and showing it off. "And the best part? It is only visible to ponies who wish to humbly serve you, the Lord of Darkness, with all their hearts. Those who do not accept their rightful place as your subjects are forever denied its magnificent beauty."
At this, the other designers looked at each other, and filled with fear, fell all over themselves praising the majesty of the phantom suit. Sombra, suitably impressed, declared the unicorns the winners. He sent heralds throughout the empire, who praised the new suit's superlative qualities (conspicuously leaving out the requirements for appreciating it) and ordered everypony to attend a parade in which he could show it off.
In the front row of that crowd stood a young Clyde. When Sombra sauntered out in front of him wearing nothing at all, Clyde's jaw dropped.
Sombra stopped and walked up to the colt. "Is there something … wrong with my clothing, my little pony?" he said, lips curling into an eager grin.
Clyde thought quickly, realizing what the unicorns must have done. "Quite possibly so, your magnificence," he said. "May I explain?"
"Oh," Sombra said, beckoning forth his soldiers, "please do."
"Your heralds said this beautiful suit was made of fabric lighter than air and yet stronger than steel," Clyde said. Sombra nodded, and Clyde pointed to the imperial guards. "What's wrong with your suit is that there's not enough of them. If your guards' armor were made of the same fabric, they would not only be nearly invincible, but also unencumbered by the weight of armor, and the grandeur of their approach would break the spirit of the rebellion before they ever lifted a hoof."
Sombra's eyes widened. "You're right." He wheeled and pointed at the tailors. "You! I'm assigning you this year's entire military budget. You are to outfit every soldier in my kingdom with new armor. Immediately."
And that is why, when Celestia and Luna led an army from the south to free the Crystal Empire, they reached the capital without losing a single battle.