If Mongolia always invaded China in the past, how come they're so weak now?

Now Mongolia has a population of less than three million. The country is very sparsely populated, with one million people living in the capital. Nobody really speaks their language. On the other hand, China now has the third-strongest economy in the world. How did Mongolia go from being such a huge threat that the emperor ordered a wall to be built between the two countries to the desolate wasteland that it is today?