What effect did foreign peoples have on the success or failure of Chinese dynasties?

Foreign people to China effected the success and downfall of the Chinese dynasties.

Why? The Foreign peoples could have come in whenever they wanted to and wage a full blown war against China and China would be unprepared, and China would soon have met its downfall. If the foreign peoples decided not to wage war on the Chinese Empire, then the Empire would be successful.

Thus, the foreign people to China effected the success and downfall of the Chinese dynasties.