As North China and Mediterranean Europe slid into population decline and urban decay from epidemics and violence, India and Southwest Asia thrived economically and culturally. The Parthian and Sassanian kingdoms of northern Iran deployed armored cavalry defenses against steppe raiders and intensified agricultural output and trade networks between the Persian Gulf and the Indian Ocean. The Gupta dynasty intensified rice paddy cultivation, spice production, and textile manufacturing in India. Meanwhile, Hindu and Buddhist holy men traveled along India’s trade routes, attracting followers in Southeast and Central Asia. The balance of power shifted again with the reunification of China under the Sui dynasty and the emergence of Islam. China’s completion of the Grand Canal and the reaffirmation of Confucianism solidified imperial power, and tributes paid to steppe peoples spread Chinese goods into Central Asia. Islam ushered in an intense intermingling of religion with war and politics and enhanced the web of trade across great distances.

Better shipping and caravan technologies, the rise of banking and coinage, and the spread of agricultural technologies drove extension and tightening of the web. Universal, portable religions were likewise integral in tightening the web. Common features of the big four religions that explain their role in the