China is unique in that it has an unbroken history and culture dating back four thousand years; no other civilization has lasted half so long. However, this does not mean their history has been peaceful. Warfare was common both within and outside their borders.
By far the most famous invaders of China were the Mongols led by Genghis Khan, beginning in 1235 and lasting until 1279, when China was finally subjugated. This made China part of the "Great Yuan," a huge empire covering much of eastern Asia. Never before had the Chinese been totally taken over by an outside power, and it is remembered as a great humiliation in national history. However, it lasted just 150 years, as the Black Death and infighting led to a total collapse of the Mongol Empire.
The next invasion came from the Manchurian Qing, who launched their own dynasty over a process that took nearly the entire 1600s, eventually bringing all of China under their control and establishing the Qing dynasty, who ruled the fourth-largest empire in human history for the next three hundred years.
Lastly, starting in 1931, the Japanese invaded Manchuria and captured cities along China's east coast. The next fifteen years would see a huge chunk of China's food, metal, energy, and money siphoned off to Japan's military as they built an empire throughout the Pacific.