How to defeat the strongest military in the world

At the height of the Mongol Empire, one of their invasions was violently arrested.

Adapted as they were to the open steppe, they were mired in thick jungles and dense swamps filled with disease, venomous snakes, and insects.

This was their foray into Vietnam.

At first, they defeated the armies they met and took a major city.

But then they were trapped in the jungle.

Which weakened their force, enough for them to be trapped on a river.

Like fish in a weir, they were harvested.