"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.' Less well known is why the Agency's analysts were so doubtful, especially because CIA was all the while a central player in US operational efforts to create and strengthen South Vietnam. Thus, it is important to examine the sources of CIA analysts' doubts about successive administrations' repeated assurances and claims."
Center for the Study of Intelligence: https://www.cia.gov/library/center-for-the-study-of-intelligence/index.html
Studies in Intelligence (Semiannual Edition 1997), no.1