Why are we Han Chinese bullied by northern nomads again and again?

Turning to the history of China, there is a strange phenomenon, that is, every few hundred years, nomadic people in the north will attack the Central Plains. If the Central Plains dynasty is strong, they can't stand any big waves, such as the Huns and Turks in the Han and Tang Dynasties. If they unfortunately encounter the weakness of the Han regime in the Central Plains, they will make great strides and occupy a part of the Central Plains, and the Han regime will be forced to move south, leaning towards the south of the Yangtze River, and five flowers will bloom in disorder, and the shame of Jingkang will form a confrontation between the north and the south. What is more serious is to eliminate the Han regime and unify China, such as the Yuan and Qing Dynasties. According to the truth, the Han nationality is far more advanced than those conquerors, and the number is far higher than that of the conquerors. Why does this often happen? Why can't we Han people walk out of the historical fate of being conquered once in hundreds of years? I think apart from the corruption of the Han regime, it is also related to the gradual loss of the martial spirit of the Han nation, but there are three more main reasons. One is that we Han people lack strategic vision, blindly guard the old site, blindly emphasize passive defense, lack positive enterprising spirit, and don't know that attack is the best defense. The Great Wall is proof. The strongest fortifications will never be breached If there are traitors in it, it will be easier to break it. The Qing army was taken away from Shanhaiguan by Wu Sangui. Even without the mole, we are still very passive. We defend ourselves on all fronts. As long as the enemy breaks through, we will be in a passive position on all fronts. You can't build an impregnable defense line everywhere, and you can't send heavy guards everywhere. The annual defense expenditure is a big burden on the national finance. In the long run, once the country encounters natural and man-made disasters, there will be a financial crisis that is unsustainable. The country will be dragged down one day, so it will be beaten one day; Secondly, when our nation was strong, it failed to develop large-scale immigration to the conquered areas in the north in time, change the ethnic structure of local residents, assimilate local ethnic groups and make it a Chinese cultural area. So although we won, we didn't get much tangible benefits, and the fruits of victory were not consolidated. This is the biggest mistake, and the hidden danger will be shown one day if it is not eliminated. During the Han and Tang dynasties, the defeated ethnic minorities moved inward, which led to the rise of Wu Luan and Qidan, and made room for the development of other ethnic minorities in the northern grassland. Third, we Han people are facing the whole nomadic people in the north, which I will call Tungusic people for the time being. We have defeated one ethnic group after another, but one ethnic group is rising. They launched attacks on our nation again and again, bravely attacked at the first time, and retreated to the grassland after defeat. We are tired of coping, and our strength is constantly consumed. Our nation can't be forever strong. Besides, the most powerful nation can't stand repeated attacks. One day we can't bear it anymore, and the disaster of our nation will come. Many people criticized the Song Dynasty. In fact, in the Song Dynasty, we Han people fought bravely (but it was undeniable passive resistance). Finally, faced with the situation of being unable to return to heaven, hundreds of thousands of Song Dynasty officers and men committed suicide by jumping into the sea. Historically, there was no China after Cliff Mountain. How tragic it is, it makes people sigh and tears fill their eyes. We were dragged alive through the Song Dynasty, first Liao, Xixia and Xixia. The demise of the Song Dynasty interrupted the normal historical process of the Han nationality, and the conquest of China by the Manchu Dynasty completely distorted the national character of the Han nationality, which remains to this day.