Look, I'm going to level with you: the Romans perfected time travel. Did it some time around 107 BC. Somehow, between reorganizing the army and becoming consul, Marius found the time, no pun intended, to develop mass, practical chronotransit.
Yes, we have proof. No, you're not cleared for it and you wouldn't believe me if I showed it to you. No, it has nothing to do with the Library of Alexandri — Look, the point is they never got around to using it and about a thousand years later the device had made its way to Baghdad when the Mongols invaded.
You haven't heard of the great Mongol Time Armies because Genghis didn't see too many applications for the thing. Took too long to figure out how to operate, for one; and besides, why mess around with barbarian technology when a horse would take you anywhere you wanted to go? But when the Great Khan died — after falling off a horse of all things — he asked for an unmarked grave, and by God an unmarked grave is what he was going to get. His most trusted adjutants started looking for a burial spot nobody'd find, somewhere suitable for a really eternal rest. That's when some enterprising captain turned to the machine, to find a future where the Khan had been forgotten. Kinda makes sense — Genghis could rest all the better if they let Time do the hard work of erasing his name first.
Rule number one of this post is that we don't know a lot about these guys — where or when they've been, or are going; how they've survived; even how they've been navigating all this time. Mongols weren't much for writing things down and as the hordette has never stayed in the timeline for more than twenty-four hours we've never gotten ahold of one to ask 'em anything. We know the departure was successful because they keep making social calls. Jumped in three times so far, in 1631 outside Magdeburg and again in Japan during the Warring States business. Left pretty quick both times, presumably as soon as they figured out that the Mongols weren't forgotten. Our only first-hand information comes from the Ulaan Baatar sighting in '85; that's how we know they've learned to read.
Containment has two parts. Each arrival emits a specialized radiation that the scientists upstairs monitor; if they manifest, an MTF'll go check things out, but don't hold your breath. Second part is keeping Genghis' memory alive so that they don't stop roaming — that's where you and I come in. We're low key — it's accounting work, really — sponsor a blockbuster here, fund a grant there. We've got about twenty million dollars in our endowment but don't let that go to your head. Director Sarangerel informs me that upper management would love to give us the axe, liquidate our budget and assign the case to General Monitoring. That nobody is less likely to be forgotten than Temuujen.
Maybe they're right. The Foundation deals with a hundred critters a day more likely to do more damage than the Little Horde That Could. I say, go and read about the sack of Baghdad and the fall of the Tangut Empire, the Battle of Liegnitz and the depopulation of Persia — not to mention the destruction of Magdeburg, which I know our boys had a hand in even if there're no records of it. You ask me, it's not the public we're here to remind.
Eventually the Horde is going to stop — if the machine doesn't break, they'll find the oblivion they're looking for or get tired of wandering and stop anyway. Think two hundred Mongols couldn't cause too much trouble anyhow? Remember, we're talking about two hundred of the fiercest and smartest of the Horde which conquered everything from Korea to Kiev, who broke empires with bows for their Khan when he was alive, and are journeying hundreds of years for him now that he's dead. Who have no fear of the future and know just what our world has to offer. Who have half a million loyal soldiers waiting back in 1227 and access to a time machine.
You think they'll just go on home after a trip like that?