At the end of Mists of Pandaria, when Wrathion agonizes over Varian refusing to conquer Orgrimmar and the other Horde settlements, believing that only a world united could properly withstand the Burning Legion’s future invasion, Tong the Fixer says something very interesting in hindsight:
"Always you speak. Never do you listen! You ignore the lessons of Pandaria! You see, there is balance in all things. Wisdom etched in our very fur: Black and white. Darkness and light.
When the last emperor hid our land from the rest of the world, he also preserved the homeland of our ancient enemy, the mantid. Why did he do this? He did so to keep the land whole. Living with the mantid for ten thousand years has made us both STRONG.
So it is with your Alliance and your Horde. They are not strong despite one another; they are strong BECAUSE of one another. You mistake your greatest strength for weakness. Do you see this?"
Sa’at, a Keeper of Time at the Black Morass also states that: “Indeed, if the orcs never make it to Azeroth, the Alliance will never form. Eventually civil war will break out, further weakening the human kingdoms. Ultimately the Burning Legion will invade Azeroth, destroying it in the absence of any defenders.”
Many are tired of the faction war, and understandably so, it is a pointless, cyclic conflict in many ways. But this statement made me pause and think for a moment. Say if the Alliance conquered all of Kalimdor, or the Horde took the Eastern Kingdoms, one faction basically dominating all of Azeroth.
Without a suitably strong enemy to constantly test their skills against in perpetual peace, wouldn’t they inevitably stagnate or even fall to in-fighting over time? Even though the pandaren strive for harmony, even in isolation, they did not become total pacifists for a reason.