Why was China never invaded?

This is an inaccurate assumption. China has been invaded and occupied by foreign forces on multiple occasions throughout its history. Some notable examples include:

- The Mongol invasion in the 13th century, which resulted in the establishment of the Yuan Dynasty.

- The Manchu invasion in the 17th century, which led to the Qing Dynasty.

- The Eight-Nation Alliance invasion in 1900, which was a joint military action by eight foreign powers in response to the Boxer Rebellion.

- The Japanese invasion of China in the 1930s and 1940s, which was part of World War II.