After Genghis Khan unified the steppe tribes in 1206, the Mongols built the largest contiguous land empire in history, stretching from Korea to Eastern Europe before fragmenting into successor khanates by 1368. They secured the overland Silk Road through the yam, a relay-post system of horses and stations that allowed messengers and merchants to cross Eurasia under common protection. This relative safety, the so-called Pax Mongolica, sped the movement of goods, people, technologies and ideas, as well as disease, across the continent. Mongol campaigns were also extraordinarily destructive, sacking cities such as Baghdad in 1258.
The Pax Mongolica intensified trans-Eurasian exchange that enriched caravan and port cities along its corridors. Yet the same conquests razed and then rebuilt many cities, reshaping the urban map from China to the Middle East.