What if, part 4
The last decades of the 12th century are filled with a terrible tension. While the Romans and Mongols engage in trade and maintain superficially diplomatic relations, the Romans can't help but feel that the Mongols are checking them out and seeking their weaknesses, for that is exactly what the Romans themselves are doing to the Mongols. There are numerous "incidents" and "accidents" -- always between distant Roman outposts and Oriental "pirates" and "bandits," and always quickly papered over with diplomacy and reparations -- and while the Romans can't help but feel proud of the way their navy swats pirates like flies, they also can't help but notice the way that Roman-trained local militia are easily overrun by bandit cavalry, nor the way in which the Mongol Empire is rapidly gulping down all of its nearby Asian neighbors. In the Imperial Senate, the debate over the Oriental Question and expanded funding and new weapons for the legions drags on for tedious years, with neither conclusion nor commitment.
The Mongol strike, when it finally comes, is terrifying in its swiftness and brutality. In a matter of months the Mongols overrun and destroy the Khwarezmid (Persian) Empire, along the way seizing all the Roman way-stations in Sindh and cutting off all Roman outposts east of Oman. The Romans are stunned by the speed with which their garrisons and legions are overwhelmed; their infantry can't stand against Mongol cavalry, their armored war chariots are no match for the swift and nimble Mongol horsemen, and potentially decisive weapons such as the steam chariot and the repeating arbalest were never procured in great enough numbers to make a difference. What's worse, the Roman navy, which performed so well with ballistas and Greek fire against pirates in wooden sailing vessels, is completely flummoxed by the appearance of armored "turtle" ships, mounting gunpowder cannons and ably crewed by the Khan's loyal Korean vassals.
The Roman defense of Baghdad is a futile disaster that serves only to make the massacre in Damascus worse. Within two years major Mongol hordes are deep into Anatolia and Rus, smaller groups are extending feelers around the northern coast of the Black Sea into Hungaria, and a third great horde is pushing towards Alexandria. Faced with the prospect of losing both the Suez and the Red Sea, Roman naval strategists begin to wonder: is it true that by sailing west from the Azores, they can circle the world and launch a strike against the Mongols unprotected eastern flank?