The guy above is correct.

1) Only idiots would try to conquer the entire China. It was extremely hard to maintain. For example, the Mongols had to chase the last Song emperor all the way down to the Guangdong China by a massive naval fleet. The Qing dynasty had to constantly watch out for possible secessions in western China like Tibet and Xinjiang. Do you know why China never bothered conquering other countries and why naval exploration was banned in the Ming dynasty when China could've easily become a Spain, UK, etc.? Because governing China was already handful from the inside, they cannot be bothered with the outside (the outside that was culturally backwards This is why China looked down on foreign powers, because their actions were considered "inferior"  according to Chinese philosophy (Confucianism & Taoism). What does that say of foreign powers who wanted to come in?

2) The foreign powers may have semi-colonized cities like Guangzhou Beijing Shanghai Tianjin but regardless of how many Chinese that were killed or put down, they always kept coming back, looking for trouble. In addition, the communist guerilla fighters gave the Japanese hell.

3) Han Chinese are culturally one people regardless of whether they are from the North/South wherever. Even Tocqueville in his Democracy in America mentioned the Chinese as ethnically "unconquerable" (this is paraphrased). Even though the Chinese have fallen behind in terms of technological advances by the end of the Song dynasty, their culture was what made them strong. The Qing dynasty, in order to survive, had to mold themselves to Han culture. For example, already into the 4th Qing emperor's reign (Yongzheng), barely any of the ethnic Manchu officials can speak Manchurian. This was the only way foreigners could control China, by ending up Chinese themselves. Ironic. In relation to number 1, the Han Chinese are very proud of their civilization. Why do you think China is called the middle kingdom? They do not care about Korea, Burma, Vietnam, Japan etc., let alone a bunch of faraway Europeans.

FYI, understanding number 3 is crucial to doing business with the Chinese. ;)