Tibet was under control of the Qing Dynasty (i.e. China) until its end in 1912.

From 1912-1950, Tibet (due to its remoteness) acted as an de facto independent region, despite Western legal precedent stating it was still under the control of Beijing.

When attempting to get de jure independence from China in 1951, China asserted control over the region.

So sort of, but not really.

So if you occupy country for long enough it’s ok to invade it when it tries to get independent?

Nobody likes to admit it, but yeah. It all boils down to might makes right; if you revolt or otherwise cause a ruckus and win then it becomes the right path of history - see: USA, etc.

You’re literally describing the US civil war.