B. A single communist nation was created when North Vietnam invaded South Vietnam.
Following the United States withdrawal from South Vietnam in 1973, South Vietnam was left to fend for itself against the invading North Vietnamese Army and their logistical supports from China & U.S.S.R.
It is important to note that in the years prior, the Viet Minh, a revolutionary movement found in areas of northern Vietnam, had successfully defeated the French colonial government during the First Indochina War. In the wake of the defeat, Vietnam was split into two countries, North Vietnam, which had a Communist regime and was supported by the governments of U.S.S.R, and China., and South Vietnam, which had a Parliamentary republic, and was supported mostly by the United States, and anti-communist governments.
In November 1, 1955, North Vietnam invaded South Vietnam as well as neighboring countries such as Laos, to establish a supply-chain and point-of-entry known as the Annamite Range Trail, which opened up most of South Vietnam to attacks. In August 2, 1964, the Gulf of Tonkin incident occurred, which saw to the event of North Vietnamese ships attacking USS Maddox. Following this, the U.S. congress passed the Gulf of Tonkin Resolution, which allowed then President Lyndon Johnson to have legality of open warfare in the Pacific Theatre to continue the Policy of Containment.
Following the Gulf of Tonkin resolution, the United States became a open-military ally to South Vietnam, and sent in troops to help stave off the invading North Vietnamese armies. This continued from August 1964 - January 1973, which saw to the withdrawal of US troops completely due to public backlash of the war. 2 years later, the Spring Offensive of 1975 led by the Khmer Rouge captured Saigon, which reunified Vietnam under the communist regime.
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