Post-classical history (also called the Postclassical Era) is the period of time that immediately followed ancient history. Depending on the continent, the era generally falls between the years 200-600 and 1200–1500. The major classical civilizations the era follows are Han China (ending in 220), the Western Roman Empire (in 476), the Gupta Empire (in the 550s), and the Sasanian Empire (in 651). The post-classical era itself was followed by the early modern era, and forms the middle period in a three-period division of world history: ancient, post-classical, and modern. The era is thought to be characterized by invasions from Central Asia, the development of the great world religions (Christianity, Islam, and Buddhism), and of networks of trade and military contact between civilizations.