Why & how did China take Tibet & giving trouble to the peaceful Tibetians?

In this modern world colonialism and imperialism gave way for the will of the people.Each and every country should respect its own boundaries and others thereby respecting sovereignty and territory of others .So that international peace and prosperity shall be maintained .China being a communist country is under liability to up hold the peace and sovereignty of all neighboring nations .The communist philosophy is against aggression ,annexation and colonialism .As such China itself allow Tibetans to live peacefully in their territory .The people of china must encourage the rulers to to withdraw from Tibet for ensuring peace full life for them .While the chines are living peacefully in their country they are under obligation to keep away from the territory of others .May the international community as well as UNO rise on the occasion to regain the sovereign and independence of the people of Tibet .Thanks for the international issue before the readers .