hi! not discouraging other responses, but meanwhile, you may be interested in some earlier discussions

• Why did China want Tibet/still want Tibet?

• The controversial one: How legit is China's oft mentioned historical claim to Tibet?

• Was the 1951 invasion of Tibet purely ideologically motivated?

• What is the basis behind the Tibet-China conflict?

A related question: Why did China allow Mongolia to become an independent nation but not Tibet?