Did he change his mind over the years? Yes, he did - a stance that is now verboten among "conservatives" and denigrated as "flip-flopping":
Buckley said that he now wished that the United States had never become involved in Vietnam to begin with, and added that he would still oppose the passage of the Civil Rights Act but not in the same terms or for the same reasons as he had then.
So the most influential conservative of his generation endorsed both the Vietnam and the 2003 Iraq wars, and came to regret doing both.
One of the great divides within conservatism today is between those conservatives who think war can solve problems and that government needs to be "heroic" and those who are skeptical of both arguments. These past few years have persuaded me that war, while sometimes necessary, even essential, is in fact the natural, long-term enemy of conservatism - because it enlarges government and destroys freedom and fans those emotions that most cloud skeptical reasoned judgment. Pre-emptive war, without any plans for the aftermath, may perhaps be the antithesis of every conservative impulse known to history. Pre-emptive war to seize non-existent WMDs is almost a text-book example of conservatism repealing itself.
I plead guilty in retrospect, but am reassured that changing one's mind in the wake of new facts is not a crime or a sin. It is a conservative virtue - and one that Buckley, alas, didn't share with most of his successors. Can you imagine Hugh Hewitt ever conceding that he was once mistaken?