Yuan Dynasty (AD 1271–1368)
The Jurchen-founded Jin Dynasty was defeated by the Mongols, who then proceeded to defeat the Southern Song in a long and bloody war, the first war in which firearms played an important role. During the era after the war, later called the Pax Mongolica, adventurous Westerners such as Marco Polo travelled all the way to China and brought the first reports of its wonders to Europe. In the Yuan Dynasty, the Mongols were divided between those who wanted to remain based in the steppes and those who wished to adopt the customs of the Chinese.
Kublai Khan, grandson of Genghis Khan, wanting to adopt the customs of China, established the Yuan Dynasty. This was the first dynasty to rule the whole of China from Beijing as the capital. Beijing had been ceded to Liao in AD 938 with the Sixteen Prefectures of Yan Yun. Before that, it had been the capital of the Jin, who did not rule all of China.
Before the Mongol invasion, Chinese dynasties reportedly had approximately 120 million inhabitants; after the conquest was completed in 1279, the 1300 census reported roughly 60 million people. While it is tempting to attribute this major decline solely to Mongol ferocity, scholars today have mixed sentiments regarding this subject. Scholars such as Frederick W. Mote argue that the wide drop in numbers reflects an administrative failure to record rather than a de facto decrease whilst others such as Timothy Brook argue that the Mongols created a system of enserfment among a huge portion of the Chinese populace causing many to disappear from the census altogether. Other historians like William McNeill and David Morgan argue that the Bubonic Plague was the main factor behind the demographic decline during this period. The 14th century epidemics of plague (Black Death) is estimated to have killed 30% of the population of China.