Going back to about 600ad and this incident in Korea:
https://en.wikipedia.org/wiki/Battle_of_Salsu
300,000 dead Chinese would disagree about Korea being part of China.
But the point of that war was that the Sui dynasty was attempting to re-assert its control of Korea (and even then China did hold on to much of Korea anyway), which had been customary under several dynasties for many hundreds of years.
And Korea again fell to the subsequent Tang dynasty (668 AD).
The point about holding Korea did not imply that China held it continuously or always completely.