the Manchurians should be satisfied with just raiding the Chinese northern borders and invading nearby cities from time to time, instead they took advantage of the chaos in Ming then and invaded China proper. did they use Ming and China to refer to country and land respectively back then? the Mongols was a junior partner in the Manchu conquest.
China had problems with these barbarians - Huns, Mongols, Manchus...since day one...since Qin and built the great wall to the high days of Han.
There were always stories of great generals leading expeditions to rout the Huns / foreign invaders. In fact up till 20th century, the Chinese were fighting enemies from the same continent (after that Japs).
Such expeditions drain resources and these outer tribes were playing a gureilla warfare - much like how US now hunt Taliban in Afghanistan. The Chinese emperors either appease them - by making peace with them - after they realise the problem cannot be eradicated by hunting them during the Qin & Han Dynasty, they began to pacify them by gifts / treating them with respect - and they reciprocate...giving Middle Kingdom many years of peace.
Its during the waning period of the dynasties that these outer tribes see the opportunity to grab power. But it takes 2 hands to clap, i supposed, there must also be increased oppression by Chinese ./ reduced benefits to these tribes that they began to create trouble.
So,the Chinese dynasties themselves are partially to blame...infighting (many a times due to dukes amongst themselves and the falling of power into the wrong hands eg: eunuchs)