How did vietnam become a single communist nation?

Explanation:

US President Richard Nixon had instituted a policy of "Vietnamization" in regard to the Vietnam War, which  emphasized that the United States needed to empower South Vietnamese forces to assume more combat duties.  He proposed drawing down US involvement in the war and seeking "peace with honor," as he put it.  By the time that President Nixon and US policy shifted to this sort of approach, it was too late to stave off the victory of the North Vietnamese and Viet Cong forces.  The US eventually withdrew its forces from Vietnam in 1973.  By 1975, Saigon, the capital of South Vietnam, fell to the North Vietnamese communist forces.  South Vietnam unconditionally surrendered to North Vietnam on April 30, 1975.

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its main object was the establishment of "settlement houses" in poor urban areas, in which volunteer middle-class "settlement workers" would live, hoping to share knowledge and culture with, and alleviate the poverty of, their low-income neighbors.