Genghis Khan, the founder of the Mongol Empire, is considered one of history’s most successful military leaders. While he didn’t conquer modern “countries” as we understand them today, his empire spanned much of Eurasia.
Genghis Khan did not conquer “countries” the way we think of countries today, because modern borders did not exist in the 1200s.
A good answer is:
Genghis Khan personally conquered or brought under Mongol control large parts of about 10–12 modern-day countries. These include areas of today’s Mongolia, China, Kazakhstan, Kyrgyzstan, Uzbekistan, Turkmenistan, Tajikistan, Afghanistan, Iran, and parts of nearby Central Asian/Caucasus regions.
By the time he died in 1227, his empire stretched roughly from the Pacific Ocean to the Caspian Sea.
But the Mongol Empire after Genghis Khan, expanded by his sons and grandsons, eventually reached much farther: into Russia, Eastern Europe, Korea, the Middle East, and Southeast Asia. At its height, it became the largest contiguous land empire in history, covering around 11–12 million square miles.
So the simple version:
Genghis Khan himself: about 10–12 modern countries’ worth of territory.
The Mongol Empire after him: 30+ modern countries’ worth of territory.
The Mongol Empire was the largest contiguous land empire in history, stretching from the Pacific Ocean in the east to the edges of Europe in the west, and from Siberia in the north to the Persian Gulf and the Himalayas in the south. Genghis Khan’s campaigns were focused on subjugating key regions, which later expanded further under his successors.
