China was a nation-state that existed on Earth before the formation of the United Earth government in the mid-22nd century.
In the 4th century BC, Chinese philosopher Sun Tzu wrote The Art of War. (TNG episode: "The Last Outpost"; ENT episode: "Divergence")
In 1839, China and Great Britain declared war on each other in what became known as the Opium War. (ST novel: Federation)
In 1930, James T. Kirk fails miserably at convincing a New York City police officer and passersby that Mr. Spock is from this country. (TOS episode: "The City on the Edge of Forever")
In 1932, seven women living in the Chinese city of Shanghai became victims of the Redjac entity. (TOS episode: "Wolf in the Fold")
In 1965, physicist Changming Weng defected from the People's Republic of China to the United States of America. (TOS - Star Trek: Assignment: Earth comic: "Brighter Than a Thousand Suns")
In 1972, Mao Zedong, chairman of the Chinese Communist Party, received US President Richard Nixon in the capital of Beijing. (TOS novelization: Star Trek VI: The Undiscovered Country; TOS - Star Trek: Assignment: Earth comic: "Too Many Presidents")
At some point in the 20th century, Gary Seven, Isis and Roberta Lincoln averted the assassination of Chairman Mao while he visited the Great Wall of China. (TOS novel: Assignment: Eternity)