abandonment of cities, and a northward shift of population. The Postclassic period saw the rise of Chichen Itza in the north, and the expansion of the aggressive Kʼicheʼ kingdom in the Guatemalan Highlands. In the 16th century, the Spanish Empire colonised the Mesoamerican region, and a lengthy series of campaigns saw the fall of Nojpetén, the last Maya city, in 1697. The Aztec Empire was formed as an alliance of three Nahua city-states: , , and . from the victorious factions of a civil war fought between the city of and its former tributary provinces. These three city-states ruled the area in and around the Valley of Mexico from 1428 until the combined forces of the Spanish and their native allies under defeated them in 1521. Despite the initial conception of the empire as an alliance of three self-governed city-states, quickly became dominant militarily. By the time the Spanish arrived in 1519, the lands of the Alliance were effectively ruled from , while the other partners in the alliance had taken subsidiary roles. The Tarascan state was the second-largest state in Mesoamerica at the time. It was founded in the early 14th century. Asia When China entered the Sui Dynasty, the government changed and expanded in its borders as the many separate bureaucracies unified under one banner. This evolved into the Tang Dynasty when Li Yuan took control of China in 626. By now, the Chinese borders had expanded from eastern China, up north into the Tang Empire. The Tang Empire fell apart in 907 and split into ten regional kingdoms and five dynasties with vague borders. Fifty-three years after the separation of the Tang Empire, China entered the Song Dynasty under the rule of Chao K'uang, although the borders of this country expanded, they were never as large as those of the Tang dynasty and were constantly being redefined due to attacks from the neighboring Tartar (Mongol) people known as the Khitan tribes. The Mongol Empire emerged from the unification of several nomadic tribes in the Mongol homeland under the leadership of Genghis Khan (–1227), whom a council proclaimed as the ruler of all Mongols in 1206. The empire grew rapidly under his rule and that of his descendants, who sent out invading armies in every direction. The vast transcontinental empire connected the East with the West, the Pacific to the Mediterranean, in an enforced Pax Mongolica, allowing the