History.com reports that:

This week in 1967, Reverend Martin Luther King, Jr., led a march of 5,000 antiwar demonstrators in Chicago. King first began speaking out against armed intervention of Vietnam in the summer of 1965. In addition to his moral objections to the war, he argued that involvement in waging war was fiscally irresponsible. He was strongly criticized by other prominent civil rights leaders for attempting to link civil rights and the antiwar movement.

Here are some excerpts from a speech he made that day:

Poverty, urban problems and social progress generally are ignored when the guns of war become a national obsession. When it is not our security that is at stake, but questionable and vague commitments to reactionary regimes, values disintegrate into foolish and adolescent slogans.
Those of us who love peace must organize as effectively as the war hawks. As they spread the propaganda of war, we must spread the propaganda of peace. We must combine the fervor of the civil rights movement with the peace movement. We must demonstrate, teach and preach, until the very foundations of our nation are shaken. We must work unceasingly to lift this nation that we love to a higher destiny, to a new plateau of compassion, to a more noble expression of humaneness.