In 1962, McNamara supported a plan for mass spraying of the rice fields with herbicides in the Phu Yen mountains to starve the Viet Cong out, a plan that was only stopped when W. Averell Harriman pointed out to Kennedy that the ensuing famine would kill thousands of innocent people.
https://en.wikipedia.org/wiki/Robert_McNamara#cite_ref-FOOTNOTELangguth20001963_48-0