The Mongols were able to build such a vast empire mainly because they had a very large and skilled military. The Mongols Created such a huge empire by killing women and children, and wiping out armies, towns, and cities. They eventually ruled almost all of Asia and most of Europe. In document 2 it states “forcing the Cathayan army to retreat, killing the finest and most courageous soldiers of Cathay, the Jurchin and Khara Khitan fighters, slaughtering then along the side of Chu-yung Kuan so that their bodies lay piledup like rotten logs.” This quote proves that the Mongols wiped out the armies.
The empire began to split as a result of wars over succession, as the grandchildren of Genghis Khan . They unified the Asian continent, and ended the Silk Road trade route in favor of established roads with a postal service. They communicated across the entire empire, and established the first paper currency economy backed by the Khan's silver. It was the largest contiguous land empire in history, although in overall square miles, The Mongols destroyed the Islamic Caliphate, and there has been no centralized Islamic authority since. They ended the Muslim Golden Age, destroying much of what was great, though ironically the Ilkhanate one of the four Khanates ,would eventually convert to Islam itself about a century after it was conquered.
The Mongol Empire had a lasting impact, unifying large regions, some such as eastern and western Russia and the western parts of China remain unified today, a bit under different ruler ship. The Mongols, except the main population, might had been assimilated into local populations after the fall of the empire, and some of these descendants adopted local religions for example, the eastern Khanates largely adopted Buddhism, and the western Khanates adopted Islam, largely under Sufi influence.
Finally, after conquering much of Asia many of Asia's trade routes now lie in the Mongol's hands. The Mongols taxed the products traded over these trade routes (Silk Road) and as a result grew wealthy.