Why you "lol" ??? Let me tell you this, for every defeat of every nation, there are both internal and external causes. The external cause is from the outside invader while the internal cause is from within ( impotent leaders, corrupt government, social disparity etc..). That is true for every nation, not just China. But any excuse can not get rid of the fact that the nation did fail.
History textbooks from any country are definite garbage. If you want to talk about Chinese History, please at least read one detailed history book first. Especially discuss with Chinese about Chinese history on the internationl forum. Because 99.9% questions had already been discussed and argued in native forum for many times.
So about your question, "the Prince" had already gave a convienced answer by comparing the regimes of mediaeval Turk and France about 500 years ago
"The examples of these two governments in our time are the Turk and the King of France. The entire monarchy of the Turk is governed by one lord, the others are his servants; and, dividing his kingdom into sanjaks, he sends there different administrators, and shifts and changes them as he chooses. But the King of France is placed in the midst of an ancient body of lords, acknowledged by their own subjects, and beloved by them; they have their own prerogatives, nor can the king take these away except at his peril. Therefore, he who considers both of these states will recognize great difficulties in seizing the state of the Turk, but, once it is conquered, great ease in holding it."
"The causes of the difficulties in seizing the kingdom of the Turk are that the usurper cannot be called in by the princes of the kingdom, nor can he hope to be assisted in his designs by the revolt of those whom the lord has around him. This arises from the reasons given above; for his ministers, being all slaves and bondmen, can only be corrupted with great difficulty, and one can expect little advantage from them when they have been corrupted, as they cannot carry the people with them, for the reasons assigned. Hence, he who attacks the Turk must bear in mind that he will find him united, and he will have to rely more on his own strength than on the revolt of others; but, if once the Turk has been conquered, and routed in the field in such a way that he cannot replace his armies, there is nothing to fear but the family of the prince, and, this being exterminated, there remains no one to fear, the others having no credit with the people; and as the conqueror did not rely on them before his victory, so he ought not to fear them after it. The contrary happens in kingdoms governed like that of France, because one can easily enter there by gaining over some baron of the kingdom, for one always finds malcontents and such as desire a change. Such men, for the reasons given, can open the way into the state and render the victory easy; but if you wish to hold it afterwards, you meet with infinite difficulties, both from those who have assisted you and from those you have crushed. Nor is it enough for you to have exterminated the family of the prince, because the lords that remain make themselves the heads of fresh movements against you, and as you are unable either to satisfy or exterminate them, that state is lost whenever time brings the opportunity."
this can be a reference for understanding the ancient China.
that is:
a united and centralized country is always giant and powerful and hard to be conquered but easy to be held.
whereas, a loose confederation is always relatively weak and inter-distrustful and easy to be conquered but hard to be held.
So, according to this logic, we can get that the primary reason of decline of China is internal, the external cause of outside invader is secondary, and this has been verified in almost each dynasty as well, if you are really familar with the Chinese history.
Whereas, most of your statement are DISARRAYED and ILLOGICAL, which prove that you are not only lack of history knowlege , but also lack of ability to understand the information from books
BTW, I have to say the theory of "the prince" is only a general model, but not a formula, and cannot explain every aspects of case of ancient China, because situation are very different between China and Europe. There are too many parameters need to be considered about if you want to adress this function named "politics".
so if you want to talk about Chinses history with Chinese people, do me a favor, go back to library and read patiently, you are too naive to discuss it now.