War in southern Vietnam (1945–1946)

The 1945–1946 War in Southern Vietnam, codenamed Operation Masterdom by the British, and also known as the Southern Resistance War (Vietnamese: Nam Bộ kháng chiến) by the Vietnamese, was a post–World War II armed conflict involving a largely British-Indian and French forces versus Vietnamese independence movements, which included non-communist nationalists, the Trotskyists, and the Stalinist-front Viet Minh, for control of the southern half of the country, after the unconditional Japanese surrender. Starting in Saigon on 23 September, the British began facilitating the return of the French to the half of Indochina south of the 16th parallel.

Western countries recognise three Indochina Wars: the first being France's unsuccessful eight-year conflict with the communist-led Viet Minh forces (1946–1954); the second being the war for control of South Vietnam, featuring American-led intervention and communist offensive, ending in 1975; finally, the intra-communist conflict, sparked by the Vietnamese invasion in 1978. This numbering overlooks the brief but significant initial conflict, from 1945 to 1946, that grew out of the British occupation force landing at Saigon to receive the surrender of Japanese forces, as well as the civil strife among the Vietnamese.

Many political activists in the former French Cochinchina, of very different political beliefs, were highly educated and influenced by Western thought, and often interacted with each another. However, the Stalinist Viet Minh leadership in Hanoi resolved to assassinate or execute all Trotskyist and nationalist figures who opposed their centralized authority. Among those targeted were Bùi Quang Chiêu, Hồ Văn Ngà, Dương Văn Giáo, Tạ Thu Thâu, Trần Văn Thạch, Phan Văn Chánh, Phan Văn Hùm, Huỳnh Văn Phương, Lâm Ngọc Đường, Hồ Vĩnh Ký, Nguyễn Thị Sương, Lê Văn Vững, and Nguyễn Văn Sâm. Almost all Vietnamese opposed the French return in August 1945, but the Viet Minh's killings of non-communist opponents eventually pushed some groups to join the French in fighting against the Viet Minh.

By March 1946, British–French forces had defeated the Vietnamese resistance, allowing the French to reassert control over southern Vietnam. Later, tensions in the north escalated into the full-scale First Indochina War.