The Mongols played an important part in early modern history, rising under the leadership of Genghis Khan to conquer and rule most of the civilzations of Asia and Eastern Europe. They conquered China, Russia, India, and most of the Asian cities of Islam. They failed to reach Egypt, Western Europe, and Japan.
The ancient Mongols were a nomadic people who in the 13th century found themselves encompassed by the world's great civilizations. China and India lay to the south, Islam to the southwest, and Russia to the west, with Europe beyond. Genghis (or Jinghiz) Khan, feared that the spread of the civilizations would destroy the Mongol way of life, and so set out to destroy them first. His goal was not conquest, but destruction. He dreamed of pasturing the Mongol horses in the ruins of the world's cities. However, once the civilizations had been conquered and Mongol rulers placed in charge, this policy was abandoned.
The Mongols employed terror as a weapon during their conquests. Entire cities were wiped out. Often, the populous would be lined up, the Mongols would pull out the skilled craftsmen to serve as slaves, and the rest were systematically slaughtered, their cleaned skulls piled into pyramids outside the walls of the empty cities. In the Persian city of Nishapur, in 1221, an estimated 1.7 million people were slaughtered. That same year in Merv, an estimated 1.3 million were killed at the hands of the Mongols, and in Meru Chahjan nearly as many again. In the city of Rayy, an estimated 1.6 million died.
In 1226, Genghis Khan launched a genocidal war against the Tanguts, a people of China. In 1258 the Mongols took Baghdad, killed an estimated 800,000 people, and destroyed the Abbasid dynasty. Kublai Khan, the Mongol emperor of China, murdered a reported 18 million Chinese over a forty year reign. What's worse, he invented central banking.
Before the rise of the Mongol empire, the Chinese and Islamic civilizations surpassed the West in power and sophistication. When Mongol power finally waned, Western Europe had surpassed the east. The destruction wrought by the Mongols on the Eastern civilizations, and Western Europe's immunity to it, likely played a large part bringing about this change.
Today, there are over 2.5 million Mongols living in Mongolia. They are, by and large, much nicer than their genocidal ancestors, though Genghis Khan is still considered a national hero.
The term Mongol is also used in an impolite and disparaging manner for people with Down syndrome.