Underestimating an adversary’s will to win can be a costly mistake in war, as Director of National Intelligence James Clapper noted in an interview last week. He said the U.S. had made that error recently in assessing the Islamic State, just as it did nearly 50 years ago in evaluating the staying power of the Viet Cong.

Clapper’s comment was part of a broader trend over the past week in which senior military and intelligence officials have been unusually forthright about issues involving President Obama’s strategy for combating the Islamic State. In addition to Clapper’s remarks, this “push back” has been evident in comments by Gen. Martin Dempsey and Gen. Ray Odierno, the chairman of the Joint Chiefs of Staff and chief of staff of the army, respectively, about the possibility that ground combat troops might be needed in Iraq and Syria.

It’s as if senior officials, having been through the vortex of the 2003 invasion of Iraq, don’t want to make the same mistake this time of suppressing concerns or misgivings. Many military, intelligence, and foreign-service officers had doubts a decade ago about the wisdom of invading Iraq, but those worries were mostly unexpressed. Not this time.

An interesting footnote to Clapper’s comment about estimating willpower in warfare is that this issue was actually a central point of internal government debate about Vietnam during the mid-1960s. After my column last week, an intelligence official pointed me to official CIA documents that show how skeptical CIA analysts were about policymakers’ rosy expectations that their strategy in Vietnam would work.

One dark assessment of Vietnam sent to me by the intelligence official was an August 26, 1966 study titled “An Analysis of the Vietnamese Communists’ Strengths, Capability, and Will to Persist in The Present Strategy in Vietnam.” This study was originally classified “top secret,” but was declassified in 2005.

The analysts’ “principal findings” must have made policymakers squirm: U.S. bombing, the report said, “at present levels … is unlikely to diminish North Vietnam’s continued ability to provide materiel support.” Though U.S. military strikes were hurting the Viet Cong, “neither internal resource shortages not allied actions within present political parameters are likely to render the Vietnamese Communists incapable of persisting.” Communist morale had declined “but not to a point presently sufficient to force any major revision in basic Communist strategy.”

The analysts’ attempts to speak truth to power in that study and others were upsetting to President Lyndon B. Johnson and other senior policymakers. A 2007 article by former CIA official Harold P. Ford for the agency’s internal journal, “Studies in Intelligence,” summed up the contrarian opinions at Langley during the Vietnam era. “It is well documented and well known that for decades CIA analysts were skeptical of official pronouncements about the Vietnam war and consistently fairly pessimistic about the outlook for ‘light at the end of the tunnel,’ ” wrote Ford. “CIA analysts widely appreciated the fact that the enemy saw its battle as a long-range conflict and was prepared to go the distance.”

The analysts’ doubts “did not find ready response downtown because they were up against fearful odds,” noted Ford. He explained later in the study: “There was a profound hubris among top policymakers. They believed their made-in-America schemes would work in Vietnam, where similar schemes by the French had not. We would succeed because of our superior firepower.”

These old studies make haunting reading now as the Obama administration plans its war against the Islamic State. The lesson is that intelligence analysts (and military officers) need to be blunt about their doubts that a strategy will work, as configured — and even more importantly, that policymakers have to listen when these professionals express doubt that a plan can succeed within political constraints imposed by the White House.