From the piece you linked to:
Quote:
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The United States entered the war to prevent a communist takeover of South Vietnam (as part of a wider strategy of containment during the Cold War), beginning with military advisory missions in the early 1960s and escalating to full warfare with the deployment of combat units from 1965 onward. By 1973, almost all U.S. troops had left the theater and in 1975, communist forces assumed control of South Vietnam. North and South Vietnam were reunified shortly thereafter.
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