This is from the November 2, 2003 program of Meet the Press.

Complete Video and Photos
Rumsfeld: No Way To Know How Many Troops It Will Take (“It Is Unknowable”) (Small – 5 MB)
Tim Russert:
“Time magazine reports this today, that this question was asked in the closed briefing with senators, “‘What troop levels do we expect to have in Iraq a year from now?,’ asked Senator Bill Frist, the Republican leader. And with that, the Pentagon chief began to tap dance.” Do you believe that you have an obligation to tell our leaders in Congress what your best estimate is for troop levels in Iraq a year from now?”
Donald Rumsfeld:
“You know, since — any war, when it starts, the questions are obvious. The questions are: How long is it going to last? How many casualties will there be? And, How many troops will it take?
Now, those questions can’t be answered. Every time someone has answered those questions, they’ve been wrong. They have been embarrassingly wrong. I’ll use another word: They have “misinformed.” By believing they knew the answers to those questions, they’ve misinformed and misled the American people.
I made a conscious decision at the outset of these conflicts to not pretend I knew something I didn’t know. And what I have said is just that. I have said it is not knowable.
Now, if you think about Bosnia, we were told by the administration back then that the American forces would be out by Christmas. That was six and a half years ago. They’re not out yet. That was — that — the effect of that was not consciously misleading — I’m sure they believed it. They were that wrong — six and a half years wrong. I don’t intend to be wrong six and a half years. I intend to have people understand the truth, and the truth is no one knows. But why is that question not answerable?
And Bill Frist knows this. He asked it because others were interested in that question. He’s been very supportive and very complimentary of what we’re doing, and it was not a critical question at all. It was a question that should have been raised. And I said was this: The security situation on the ground is going to determine the total number of security forces that are needed in Iraq.”