Vietnam convinced elites that mass armies fueled domestic radicalization.
Evidence
The Vietnam crisis made crystal clear all the latent political dangers associated with such a dependent relationship. The American ruling classes came to recognize the full scope of the powerful dynamics of social radicalization that could be engendered by an unpopular imperialistic war. They could also see to what extent the explosive synergies activated by the antiwar movement resonated with all the social movements agitating American society. — Grégoire Chamayou, A Theory of the Drone, 597