Imperial China was 13 dynasties with long stretches of war and peace accompanied by changing borders. If you look at all the Imperial Dynasties they never occupy collectively the space the modern PRC does. The PRC has imprinted upon us that it's borders are inherited when they are not. Mao won the war and immediately attacked Tibet, Xinjiang, Russia, Vietnam, and Korea. We just gloss this over because of normalization and because for 30 years Zhongnanhai has been fighting to change maps and curriculum globally to reinforce some fallacious opinions.