After serving as defense secretary in the final stretch of the Ford administration, Rumsfeld came back to the Pentagon in January 2001 with a mission to cut the bureaucracy and modernize the military.
That mission changed, however, after Sept. 11, 2001 -- a day when he rushed through smoke-filled corridors inside the Pentagon to aid the wounded. He grabbed a mangled shard of the destroyed airliner that day from the lawn outside the Pentagon, a piece of twisted metal he has had mounted and keeps on display in his office in Washington.
After 9/11, he told Sawyer, "I had to impose a sense of urgency into the department. ... This was the first war of the 21st century."
He added: "There wasn't a guidebook or a map or some program that said, 'Here's how you do this.' We had to figure it out."
Rumsfeld had frank assessments of several of his colleagues and contemporaries. Asked if he admired President George H.W. Bush -- a Republican president he didn't serve under -- Rumsfeld was curt.
Former Defense Secretary Donald Rumsfeld sat down with Diane Sawyer for an exclusive interview about his new book "Known and Unknown."
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"No. No," Rumsfeld said. "No, I was kind of disappointed in him. ... He decided he wanted to leave people with the impression that he didn't want to go to the CIA [in the Ford administration]. And that someone made him go there. And it was probably Rumsfeld or something."
His relationship and sometime rivalry with the first President Bush, he said, left him "amazed" that his son asked him to return to Pentagon post he had left 24 years earlier.
"It shows a lot -- about George W. Bush," he said. "That he's his own man."
Rumsfeld was less impressed by some of the president's closest advisers. Of Condoleezza Rice, President Bush's national security adviser and later secretary of state, he said, "She'd never served in a senior administration position" -- a lack of experience that showed in her lack of organization in putting together critical meetings.
"She'd been an academic. And, you know, a lot of academics like to have meetings," Rumsfeld said. "And they like to bridge differences and get people all to be happy."
Colin Powell, Bush's first secretary of state, "did not, in my view, do a good job of managing the people under him," Rumsfeld said.
"There was a lot of leaking out of the State Department, and the president knew it," he said. "And it was unhelpful. And most of it ended up making the State Department look good. We didn't do that in the Pentagon. I insisted we not do it."
Powell, Rumsfeld said, never spoke up in meetings with the president to raise objections to the Iraq war.
"There's a lot of stuff [in] the press that say Colin Powell was against it. But I never saw even the slightest hint of that," he said.
He said that Powell -- along with other top Bush administration officials and advisers -- truly believed Saddam had weapons of mass destruction at the time of his famous presentation to the United Nations in February 2003. Powell would say in May 2004 that some of the intelligence he based his presentation on was "deliberately misleading."
"My Lord, he's the guy who had more experience than anyone else," Rumsfeld said of Powell. "He worked hard with George Tenet, with Condi Rice. He prepared his speech. He went up to the U.N. He made his case. And he wasn't lying. The idea that he was lying or duped is nonsense."