China has been invaded by foreign countries in past dynasties, but why not the three countries that are constantly at war?

At the end of the Eastern Han Dynasty, just a few years after the Silk Road was opened, the traffic was backward, so the Buddhist scriptures and white immigrants at that time were not too serious, there were no serious Buddhist scriptures that seriously shook the spirit, and there were no white immigrants who seriously shook the body. Therefore, a large number of traitors are not made from physical blood or spiritual thoughts. At the same time, because learning from Western learning has just sprouted from Zhang Qian, backward traffic has not been brought smoothly, and the independent research of innovation and invention has not been weakened. In addition, the surrounding small countries do not have independent innovation like modern Europeans, which leads to soft power advantages such as science and technology and art of war. In fact, in the pre-Qin period, when there was no education and no white and black immigrants, it was also a vassal civil war, but it was easy to resist barbarians.