Tonkin (tŏnˈkĭnˈ, tŏngˈ–) [key], historic region (c.40,000 sq mi/103,600 sq km), SE Asia, now forming the heartland of N Vietnam. The capital was Hanoi. Tonkin was bordered on the north by China, on the east by the Gulf of Tonkin, on the south by the historic region of Annam, and on the S and W by Laos. The region of Tonkin was conquered in 111 B.C. by the Chinese, who ruled until they were ousted in A.D. 939, at which time the area became independent. The inhabitants began a southward expansion, and by 1471 they had acquired the kingdom of Champa. After the division of the Vietnamese lands between two dynasties in 1558, the northern half was ruled from the city of Tonkin (modern Hanoi); thus the name of Tonkin came to be applied by Europeans to the whole area. The two regions were reunited in 1802 under the rule of the restored line of Hue as part of the empire of Vietnam. To open the Red River to French trade, French expeditions were sent into Tonkin in 1873 and 1882; that of 1882 resulted in a full-scale colonial war, complicated by Chinese intervention (China also claimed the region) against the French. In 1884, Annam accepted a French protectorate, conceding France a separate protectorate over Tonkin with control more direct than over Annam. In 1887, Tonkin became part of the Union of Indochina. In World War II, the region was occupied (1940–45) by the Japanese. After the war Tonkinese and Annamese nationalist leaders joined in demanding independence for the state of Vietnam, and Tonkin was torn by guerrilla warfare between the French and the Viet Minh nationalists led by Ho Chih Minh. The name also appears as Tongking and Tonking.