The Vietnam War Thesis: America's involvement in the Vietnamese Conflict, was to stop the spreading of communism in Southeast Asia. The United States played a major role in the Vietnam War. Its influence was greatest in the South, led by the Capitalist party of Ngo Dinh Diem. The French had been defeated by the Vietminh, trying to defend South Vietnam and failed to end the Vietnamese conflict. The United States was forced into the war by its anti-communist policies, which were to prevent the spread of communism in Southeast Asia. The Vietnam War took place in a time when Americans did not know what they wanted, but what they did know was that they did not want war (Doyle 95-110).