9. What physical feature might make an army invasion from Mongolia difficult?

The Gobi Desert.

Explanation:

The Gobi Desert, located in southern Mongolia as well as partially in northern China, is the fifth largest desert on the planet. It occupies a third of the Mongolian territory, and is characterized by its extremely dry climate, its altitude of 1000 meters above sea level and its almost total lack of water. This, added to its extension of almost 1.3 million square km, makes this desert become a strategic defensive barrier for Mongolia in the south of the country.