Mao Zedong planned the invasion of Tibet years before it began. He wanted to bring Tibet under Chinese control, because he thought of Tibet as a rebellious province. But this was untrue because Tibet has been independent for over 1200 years. Mao Zedong inspired the people and said that he would reconquer the cities of Chang'an and Chengdu.
China demanded in August 1950 from Tibet that the cities of Chang'an and Chengdu must immediately returned to China. Tibet refused and China declared war on October 7, 1950. China rapidly conquered Chang'an and Chengdu and later Xining and Golmud. The growing conflict drew the attention of the United Nations and the UN sent armies to assist Tibet in the war. The reason behind the Chinese victories in the early war was that Tibet abondoned most of it's armies in the 19th century. China advanced into Tibet and laid siege to various cities before besieging the great city of Lhasa. The Tibetans defeated the Chinese in a Battle for Independence. The United Nations proposed an ultimatum: Tibet was to hold Greater Tibet, Tarim Basin and Xinjiang, with China holding all other provinces. China ignored the demand and the United Nations and launched atomic bombs on the cities of Beijing and Shanghai, kiling Mao Zedong and killing many innocent people and destroying large parts of the Forbidden City. China surrendered and collapsed into civil war one year later.
One year later the government fell and the PRC collapsed into civil war. In 1961 China was reunified under the Second Zhou Dynasty by Shao Zhou.