Editor's note: Tune in Saturday, September 30 at 8 p.m. and 1 a.m. ET for the FOX News Channel special "Why He Fights."

Watch a clip from "Why He Fights"

Five years after 9/11, the debate over the United States' war against terrorism rages on. With the leak, and subsequent declassification, of parts of the National Intelligence Estimate, Americans are asking themselves if the way the Pentagon and this administration have fought the War on Terror and the War in Iraq, has in fact made the threat of Islamic terrorism even worse.

So it’s no surprise that once again Defense Secretary Donald Rumsfeld is taking a lot of flack — fending off calls for him to step down and feeling the heat from Capitol Hill for how the U.S. is fighting in Afghanistan and Iraq.

Which is why, for me, Rumsfeld continues to be one of the most fascinating figures in President Bush’s war cabinet. At 74 years old, he is a self-made millionaire many times over. He once served as the nation’s youngest defense secretary — now he’s the oldest. So what keeps him going? What makes him continue to fight?

Since 9/11, Secretary Rumsfeld has spent more time on television, speaking to the American people, than any other public official besides President Bush. He became a media star with his Pentagon briefings in the early days of the war in Afghanistan. When war in Iraq didn’t go as expected, he then became the Bush Administration’s lightning rod.

Americans have seen so much of Donald Rumsfeld that they think they know him, but I can tell you this, from personal experience: I have never covered a public figure who's been harder to crack, who’s been better at controlling an interview, and who can sound so candid while revealing so little.

That’s why, when I set out to do a documentary on Rumsfeld, I knew I’d have to try to spend as much time with him, in as many different places, over an extended period of time, as possible. That documentary, “Why He Fights,” will air on FOX News Channel this Saturday, at 8 p.m. At the heart of this hour is a series of one-on-one interviews with Rumsfeld that took place over the course of several months. I traveled with Rumsfeld to Iraq numerous times, spoke with him at the Pentagon, and even rode along with him as he traveled to and from the White House.

It was in that car ride that FOX cameras captured some of the rarest, most unguarded Rumsfeld moments that I’ve seen, including his answer to this question:

“You know we've talked many times over the past four and a half years, but I have never asked you this question,” I said. “We've heard President Bush talk about his faith and how it factors into his job. I've never heard you talk about yours. Are you a religious a man?”

Rumsfeld’s answer may surprise you. You can hear what he had to say by clicking here.

Be sure to tune in for the whole hour of “Why He Fights” on FOX News Channel on Saturday night. If you do, I doubt you will think about Defense Secretary Donald Rumsfeld in the quite same way again.

Bret Baier is the Chief White House correspondent for FOX News Channel. You can read his bio here.

Bret Baier currently serves as anchor of Fox News Channel's "Special Report with Bret Baier" (weeknights 6-7PM/ET), the top-rated cable news program in its timeslot. Based in Washington, D.C., he joined the network in 1998 as the first reporter in the Atlanta bureau. Click here for more information on Bret Baier