How large was Genghis Khan's empire?

At its height under his successors, the Mongol Empire covered roughly 24 million square kilometres — the largest contiguous land empire in history, stretching from Korea to Hungary and from Siberia to the Persian Gulf.

Genghis Khan, born Temüjin around 1162, united the fractious Mongol tribes and was proclaimed ruler in 1206. By his death in 1227 the empire already extended across much of Central Asia and northern China. His sons and grandsons continued the expansion, and the empire reached its greatest extent around 1279.

The scale is easier to grasp by comparison: it was roughly twice the area of the Roman Empire at its peak, and only the British Empire — scattered across oceans rather than contiguous — ever covered more territory.

The military system explains much of it. Mongol armies were built around highly mobile mounted archers, an unusually effective communication network, and a willingness to adopt siege technology from conquered peoples. Discipline was organised through a decimal structure, and commanders were promoted on ability rather than birth, which was unusual.

The empire's consequences were not only military. The Pax Mongolica made overland trade across Eurasia comparatively safe for roughly a century, moving goods, technologies and ideas — and, in the fourteenth century, plague. Genghis also promulgated a legal code and granted a degree of religious tolerance across his territories.

The human cost was enormous. Estimates of deaths from the conquests run into the tens of millions, and several cities were destroyed entirely.

The empire fragmented into competing khanates within a few generations, largely through succession disputes.