[The Pentagon Papers: The Secret History of the Vietnam War, The Complete and Unabridged Series as Published by The New York Times, taught me that sometimes the truth is hidden in plain sight. In this case, all I had to do was open a book, and some of the truth spilled out. Bolded text is my emphasis.]
Their report evaluating the results of the Rolling Thunder campaign began:
“As of July, 1966, the U.S. bombing of North Vietnam had had no measurable direct effect on Hanoi’s ability to mount and support military operations in the South at the current level.”
They then pointed out the reasons that they felt North Vietnam could not be hurt by bombing: It was primarily a substinence agricultural country with little industry and a primitive but flexible transport system, and most of its weapons and supplies came from abroad.
These factors, the scientists said, made it “quite unlikely” that an expanded bombing campaign would “prevent Hanoi from infiltrating men into the South at the present or a higher rate.”
In conclusion, the Pentagon study says, the scientists addressed the assumption behind the bombing program – that damage inflicted on a country reduces its will to continue fighting. The scientists criticized this assumption, the study says, by denying that it is possible to measure the relationship.
“It must be concluded,” the scientists said, “that there is currently no adequate basis for predicting the levels of U.S. military effort that would be required to achieve the stated objectives – indeed, there is no firm basis for determining if there is any feasible level of effort that would achieve these objectives.”