D) Americans questioned the containment policy.
One result of the U.S. involvement in the Vietnam War was that it led to widespread questioning and criticism of the containment policy. The Vietnam War was a protracted conflict in which the United States aimed to contain the spread of communism. However, as the war escalated and casualties mounted, many Americans began to question the effectiveness and legitimacy of the containment policy. The anti-war movement and protests against U.S. involvement in Vietnam reflected a broader skepticism and reevaluation of the country's foreign policy approach.