Political Realities
The Manehattan River Valley that served as the cradle for pony civilization was well protected, by the sea to the south, by the mountain chains to the north, east and west, and by vast uncharted territory beyond those. It was a most fortunate happenstance, because during these first few centuries of settlement and development, there were two great empires that shared the continent that would have wiped the pony tribes out to a mare without so much as a blip on their collective conscience.
The Mongrellian Horde
It is hard to believe that the packs of Diamond Dogs scattered about Equestria and the continent today are the descendants of a conquering horde whose empire covered more square miles than any other in history.
It all started in ca. 4199 BW with a Diamond Dog named Fluffy far to the Eastern desert, who united all of the Canis Tribes and declared the founding of the Mongrel state. With an ever burning desire for wealth in gems and precious metals, Fluffy’s horde proceeded to spread out in all directions, conquering civilizations left and right, enslaving many of the local populations, often ponies, as mine workers. If the stories are even half-true, the conditions in the mines were horrendous by any standard, with miners being worked to death, their bodies left to rot in the tunnels. Earth Ponies forced to pull overloaded carts up and down the tunnels; unicorns being almost totally drained of magic searching out deposits of gems, and being killed if they were unable to recover their reserves in time; and pegasi being locked in cages as living methane detectors, the physiology that made them able to survive at high altitudes and speeds also making them especially vulnerable to the poisonous gas.
Given the correlated dates, it is most likely that the nomads that would settle the Manehattan River Valley were in fact running from the ever expanding empire, stopping only when they made sure that it was safe, a condition the Valley gladly provided. Even though Diamond Dogs are excellent diggers, effective burrowing is only good for approximately half a mile in the best conditions. After that it becomes more and more difficult for the Dog to put forth the energy. The mountains that protected the River Valley are loaded with dense metamorphic rock, making effective burrowing nearly impossible, and after the Celestine Junta came into power, there was an addition military defense that made surface invasion a less appealing option. Therefore, other than a few stabs at the area, the Horde largely ignored the valley in favor of softer targets. Furthermore, the Horde had to deal daily with near constant slave rebellions, and by 2857 BW the Horde also had to deal with both the Griffin Empire, which was at its peak military might, as well as infighting over succession crises over which line of descent from Fluffy ought to rule. But the time of the Great Warming, the Horde was gone, leaving only scattered packs behind, each seeking out its own interests.
The Ancient Griffin Empire
The Griffin Empire has its roots far to the northwest, when eighty-six griffin tribes were conquered under the might of the eighty-seventh in the 36th Century BW. From there, they rose to prominence over the course of centuries, crushing smaller nations under their wings. By 3000 BW the Griffins warred with the Horde almost constantly over territory, resources and slave populations, with ponies outside of the Manehattan River Valley suffering the most from the power struggles. It seemed as though the juggernaught could not be stopped, having become too strong to be pushed back from where ever it set its sights.
However, by the 1000s BW the Empire had become thoroughly corrupt and weak from within. Those in power were either too lazy to address the problems of rampant inflation, crumbling infrastructure and barbarian invasions, or they were ambitious and lusting for personal power. In the final decades, the formerly glorious Griffin Empire was hemorrhaging territories like a wounded beast. Rogue war lords who were once loyal generals or governors and their personal armies freely fought each other and the Empire, and were winning battle after battle. Commander Hurricane commented that the Empire, once the “Ever-Looming Shadow over the Junta and her very existence, who both inspired and terrorized her dreams, is now nothing more than a sick old mare, to lame to even turn her head without stumbling. The Griffins stand now only out of the force of millennia of habit.”
The last Griffin Emperor, King Imperious XIII, was killed in 85 AW, and with him and his family dead, the Empire was no more. After the Talonhoof Assault in 100 AW, the Griffins as a race became splintered, the corpse of their Empire ravaged and balkanized, leaving only thousands of little statelets and city-states and ruins in its wake.
Inauspicious ends to be sure, but at their height the Griffin Empire and the Mongrellian Horde were indeed ‘the ever-looming shadow’ over the valley. From the stories of the few refugees who made it to the valley from the Empire and through Mongrel territory, the three tribes begrudgingly recognized the threat of the two empires, resulting in a treaty stating that if any one of the tribes were threatened, the other two would come to its defense. In the end it was not worth the parchment it was written on, for as soon as news of the empires stopped coming in, the treaty was forgotten.
The Interconnections
Even if either the Empire or the Horde considered wiping out the Manehattan Valley ponies, and there is little evidence suggesting so considering both the distances and the natural defenses of the valley, the very nature of the ponies connection to the world at large provided the largest reason to not invade.
The most obvious being the unicorns. Before the appearance of Celestia and Luna, the rising and setting of the sun and moon was reliant on their magic. The actual common knowledge of the spell involved is lost to all but a few ponies other than the Princesses, but careful study shows that the execution would require every single unicorn of the tribe pumping in so much power that it would not be uncommon for the entire tribe to be unconscious for at least an hour or more. But in spite of the obvious weakness to exploit, no sane commander or ruler would want to jeopardize their survival by enslaving or killing the only beings in the world that controlled the heavenly bodies.
Next, the Pegasi provided an equally strong deterent in their control over the weather. Any non-pegasus pony even today can feel the sort of respect one reserves for a potentially dangerous animal watching the pegasi construct a powerful storm, and that is after being exposed to their ability for one’s whole life. For a non-pony, that respect becomes stronger, and if he knows that his enemy controls one of the strongest determining factors of a battle, no sane military commander is going to want to deal with that on a large and organized scale, such as with the Celestine Junta.
And finally there are the Earth Ponies, whose strength per-pound and general stamina is the highest of any other race. Meaning that a properly trained or conditioned Earth Pony can out-fight and out-last any soldier, be they Griffin, Diamond Dog or anything else. Furthermore, with their connection to the earth and agriculture, given enough time, Earth Ponies in theory could cultivate a dense and vast system of hedges or brush that would slow even the fastest surface army down to a crawl, either buying the Earth Ponies time to escape, or leaving the army vulnerable to attack.
So, while the ponies of the Manehattan River Valley were without a doubt an easy and tempting target for any of the larger nations beyond their borders, the ponies themselves had formidable enough natural defenses that such an attack would become very unpleasant very quickly, thus settling the whole region into an uneasy understanding of Mutually Assured Destruction. This understanding was largely what kept the ponies in the valley from exploring beyond their lands. When the blizzards came and the three tribes decided to move, the major deciding factor, even more so than the cold and lack of food, was the status of both the Mongrellian Horde, which no longer existed in 10 BW, and the Griffin Empire, which was a landlocked vestigial version of itself that was crumbling around the ears of its rulers.