Answer: After December 1949
Step-by-step explanation:
There is no map attached however the above should be the answer. The Chinese Communist Party emerged victorious from the Chinese Civil War which ended after the Second World War.
Even though the Communists took over Beijing in early 1949, the Nationalists continued to fight with their leader, Jiang Jieshi only fleeing the country to Taiwan in December 1949.
After that the Chinese Communist Party then had to conquer the westernmost parts of China and only accomplished this after December 1949.