“I have a feeling that we are doing better in the war [in Vietnam] than the people have been told.”
As quoted in Los Angeles Times (16 October 1967)
1960s
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Ronald Reagan 264
American politician, 40th president of the United States (i… 1911–2004Related quotes

No More Vietnams (1987).
1980s

Conclusion
1880s, Personal Memoirs of General U. S. Grant (1885)

To Leon Goldensohn, May 2, 1946, from "The Nuremberg Interviews" by Leon Goldensohn, Robert Gellately - History - 2004.

American escalation and forcing out of Nguyen Khanh
1980s, Interview with Nguyen Khanh (1981)

"The Soldier's Faith" Memorial Day address at Harvard University (30 May 1895) http://people.virginia.edu/~mmd5f/holmesfa.htm.
1890s
Context: As for us, our days of combat are over. Our swords are rust. Our guns will thunder no more. The vultures that once wheeled over our heads must be buried with their prey. Whatever of glory must be won in the council or the closet, never again in the field. I do not repine. We have shared the incommunicable experience of war; we have felt, we still feel, the passion of life to its top.

This appears to be a variant of a widely disseminated Republican joke with no published attribution of its authorship to Buckley.
Mark Hatfield, as quoted in The Condition of Republicanism (1968) by Nick Thimmesch, p. 65
They told me if I voted for Goldwater we'd be at war in Vietnam in six months — and I did and we were.
Anonymous voter, as quoted in It All Comes Back to Me Now : Character Portraits from the "Golden Apple" (2001) by William O'Shaughnessy, p. 85
Buckley did say this on the Firing Line episode "Vietnam: Pull Out? Stay In? Escalate?" According to the transcript here http://hoohila.stanford.edu/firingline/programView2.php?programID=22, he says "...if someone told me that if I voted for Goldwater, we would escalate the war, I did and we have."
Misattributed
Variant: They told me if I voted for Goldwater in 1964, that we'd have more war and higher prices. Well, I did, and we do.

“[Iraq is] not Vietnam, we have a government that has a support of the majority of the people.”
Late Show with David Letterman, June 16, 2005
2000s

“We the people have been told there is no choice. You must buy — the IRS.”
As quoted by Seven Days. http://7d.blogs.com/blurt/2012/07/maine-gov-paul-lepage-doubles-down-on-gestapo-comment-after-brock-fundraiser.html (July 12, 2012)