The correct answer is B) the U.S. bombing of North Vietnam had been ineffective.
Robert McNamara´s change in policy regarding the war was influenced by a defense study that found that the U.S. bombing of North Vietnam had been ineffective.
Robert S. McNamara (1916-2009) was the Secretary of Defense during the U.S.-Vietnam War. During his tenure, McNamara thought that the U.S. Army had to be prepared to fight the war in any conditions, so he reorganized the United States defense program.
In 1967 he ordered a study of the role of the U.S. in Vietnam. The document was known as “The Pentagon Papers.” Disillutioned by the war results, he resigned in 1968.
