The Mongol Empire was founded by Genghis Khan in 1206. At its height, it was arguably the largest contiguous empire in human history, stretching from Southeast Asia to Europe, covering 13.8 million square miles or more than 35 million square kilometers. According to some sources, the empire comprehended almost 50% of the world population, including the most advanced and populous nations of that time: China and many of the main contemporary states of the Islamic world in Iraq, Persia, and Asia Minor.
Genghis, through political manipulation and force of will, had united the Mongol tribes under his banner by 1206. He quickly came into conflict with the Jin empire of the Jurchen and the Western Xia in Northern China, and, under the provocation of the Khwarezmid Empire, moved into Central Asia as well, devastating Transoxiana and eastern Persia, and raiding into southern Russia and the Caucasus. While engaged in a final war against the Western Xia, Genghis fell ill and died.
After Genghis’ death in 1227, his successors, under the second khan Ögedei Khan, continued the expansion. They expanded into Persia, finished off the Xia and the remnants of the Khwarezmids, and came into conflict with the Song Dynasty of China, starting a war that would not end until 1279 with the complete occupation of that country, and the assumption of Chinese rule by the Mongols. In the late 1230s, the Mongols under Batu invaded Russia, reducing most of its principalities to vassalage, and pressed on into Eastern Europe. In 1241 the Mongols may have been ready to invade Western Europe as well, having defeated the last Polish and Hungarian armies at the Battle of Legnica and the Battle of Mohi They returned home, however, to participate in the election of the next khan.
During the 1250s, Genghis’ grandson Hulegu, operating from the Mongol base in Persia, destroyed the Abbasid Caliphate in Baghdad and destroyed the cult of the Assassins, moving into Palestine towards Egypt. The khan Möngke having died, however, he hastened to return for the election, and the force that remained in Palestine was destroyed by the Mamluks under Baibars in 1260 at Ayn Jalut.
Already during the reign of Kublai Khan, the empire was in the process of splitting into a number of smaller khanates. Kublai concentrated on the war with the Song, assuming the mantle of ruler of China, while the more western khanates gradually drifted away.
Inter-family rivalry (compounded by the complicated politics of succession, which twice paralyzed military operations as far off as Hungary and the borders of Egypt, crippling their chances of success) and the tendencies of some of the khans to drink themselves to death fairly young (causing the aforementioned succession crises) hastened the disintegration of the empire.