In opposing the Vietnam War (1945–1975), Macdonald defended the constitutional right of American university students to protest the public policies that facilitated that war in Southeast Asia, thus he supported the Columbia University students who organized a sit-in protest meant to halt the university's functions. Yet as a political radical himself in 1968, Macdonald criticized the Students for a Democratic Society (SDS) organization for insufficient ideological commitment, for showing only the red flag of revolution and not the black flag of anarchism, his political taste.