Rise of Mongol Power

About 1162, there was born to a noble clan of the Mongols a child named Temuchin. He grew in prestige and power the way any charismatic individual did in that society, by success in raiding and clan warfare. It was common for charismatic leaders among Asiatic nomads to assemble short-lived confederations as large in area as the United States, only to have them disintegrate when the ruler died or lost his power. By 1206 Temuchin had done what no other tribal leader had ever done before: assemble all the Mongol tribes under a single ruler. At a ceremony in that year he was given the title Khan of Khans and the honorific name by which he is better known to history - Genghis Khan. What separates Genghis Khan (1162-1227) from all his predecessors is that Genghis extended his authority over a vast region and created institutions to perpetuate Mongol power.
China has historically existed in one of three patterns: a unified whole when the central government was powerful, at other times division into northern and southern kingdoms, and occasional periods of disintegration and civil war, called Warring States periods. When Genghis Khan came to power, China was divided into a northern Kin Dynasty and the southern Sung Dynasty. The Mongols invaded the Kin realm and raided it and Korea from 1211 to 1214 before the Kin surrendered and agreed to pay tribute.
In 1218, the event took place that would change Genghis' realm from just another nomadic confederation to a world empire. A caravan traveling from Mongol lands to the Persian Empire was stopped by the governor of a Persian frontier province in modern Uzbekistan. Suspecting, probably correctly, that the caravan included Mongol spies (the Mongols were voracious intelligence gatherers), he ordered the caravan massacred and its goods seized. To the Mongols, ambassadors and caravans under safe-conduct were inviolate, and this violation was unforgivable. Genghis sent ambassadors to the Shah of Persia demanding that the offending governor be turned over. To the Shah, ruler of a populous empire of a million square miles, this request seemed as preposterous as it would seem to us for the President of Haiti to demand that the President of the United States turn over the governor of Florida. The Shah humiliated the Mongol emissaries and put them to death, another unforgivable offense to the Mongols.
Genghis declared war, and although the Mongols took many rich cities with frightful bloodshed, they had barely touched the frontiers of the vast Persian Empire. However, Genghis made good use of the Mongol passion for accurate intelligence; he knew that the Shah's Empire was fragmented and filled with ethnic and religious groups who were held in check only by force. If the demand to turn over the offending governor was bold, what followed next was all but incredible: Genghis ordered two of his generals to hunt down the Shah in his own empire. To use the modern analogy above, imagine that the President of Haiti, having been refused, sends troops to fight their way across the United States to capture the President of the United States. Imagine further that they actually do it. The Mongols obliterated resistance when they encountered it but bypassed areas that offered none. The word soon got out that the Shah was the target, and that interfering with the pursuit was certain death. The Shah was soon in full flight for his life and barely made the Caspian Sea ahead of the Mongols. There, on an island, he died with only a few loyal followers, so poor they could not even afford a burial shroud. Mopping up operations continued until 1223. Intrigued by stories of the Caspian being landlocked, the Mongols sent a reconnaissance in force around the sea on a two-year journey (1222-1224). The Mongols carved a bloody track across Armenia and Georgia, and for the first time Europe learned of the Mongols.
The Mongol conquest of Persia had an interesting effect on Europe. There had long been a rumor of a great Christian King of the East, Prester John. The attack on Persia was thought to be the start of Prester John's campaign to help Europe destroy Islam. The rumors, interestingly enough, had a slender basis in fact. About 500 A.D. an aberrant Christian sect called Nestorianism was suppressed in the Byzantine Empire. (Orthodox Christianity holds that Christ was simultaneously human and divine; Nestorians believed Christ had two distinct personalities. If this seems subtle and irrelevant, welcome to the Middle Ages. It mattered to them.) Many Nestorians took refuge in Persia and from there diffused far across Asia. Many of the Mongols were technically Nestorian Christians, although their Christian beliefs were heavily mingled with other belief systems, and many Mongols saw no contradiction in being both Nestorians and adherents of other religions.
The Mongols were sometimes called the Tatars, which is actually a corruption of the Chinese term for one of the Mongol peoples. In Roman mythology, however, Tartarus was the Roman equivalent of Hell. Thus it's not surprising that Europeans equated the two and soon began calling the Mongols Tartars, the people from Tartarus. (The tartar on your teeth and the cream of tartar in your cupboard come from an Arabic word for a type of resin and have no connection, in case you were wondering.)