Well, I think it's very simple, because the Tang Dynasty was a world-class power at that time, and those small countries were too weak to be destroyed. For example, during the Wanli period of the Ming Dynasty, when Japan invaded Korea, korean king turned to his home country (Ming Dynasty) for help after being beaten at the border between China and North Korea. When the Ming Dynasty sent troops and Japan was defeated, North Korea had little strength (the Ming army had to pay cash to North Korea when it fought), and so did the North Korean army. But the historical evaluations made by the three countries are completely different. Japan's historical record of this is that there are many talented people, and several defeated soldiers in the Ming Dynasty have been blown to the sky. Instead of mentioning failure, it seems to be how many victories they have won. After sending away the reinforcements of the Ming Dynasty, North Korea built a big newspaper altar, hoping that future generations would not forget the kindness of the motherland (Ming Dynasty), but the big newspaper altar disappeared at an unknown time (no textual research). North Koreans' record of this history is that it was a crucial period that decided the fate of North Korea. Thanks to the hard work of North Korean volunteers, they finally saved North Korea and praised a man named Li Shunchen as a national hero. Today, the whole Korean Peninsula knows him, just as China people know Yue Fei. The reinforcements of the Ming Dynasty are only dispensable in their history. Both Japan and North Korea paid so much attention to this matter, but the Ming Dynasty was just the opposite of them. The Ming Dynasty recorded little about this period of history. It should be said that it is very stingy with pen and ink. It's not that people who write history are lazy, but that nobody pays attention to China. Among the big countries in China, only one North Korea is not as important as several chiefs in Guizhou. This is the difference between a big country and a small country. The close-up history of Japanese and Korean capital is not worth mentioning in China's eyes, which is the problem of strength. So if you look at the history of the Tang Dynasty in reverse, you will understand why big countries don't even mention the demise of small countries, because they are too weak to mention it.