Rice's much anticipated testimony was being televised nationally by CBS News, and webcast live on CBSNews.com.
Her remarks were an effort to rebut claims by former counterterrorism director Richard Clarke, who claims the Bush administration in general — and Rice in particular — ignored his warnings about al Qaeda and Osama bin Laden.
"There was no silver bullet that could have prevented the 9/11 attacks," Rice said.
"If anything might have helped stop 9/11," Rice continued, "it would have been better information about threats inside the United States, something made difficult by structural and legal impediments that prevented the collection and sharing of information by our law enforcement and intelligence agencies."
The debate over the Bush administration's pre-Sept. 11 counterterrorism strategy has hinged on whether the Clinton administration took the al Qaeda threat more seriously than the Bush team.
"America's response across several administrations of both parties was insufficient," Rice said, adding, "tragically, for all the language of war spoken before September 11th, this country simply was not on a war footing."
Rice said the Bush team received from the Clinton administration two briefings on terrorism.
"Because of these briefings and because we had watched the rise of al Qaeda over the years, we understood that the network posed a serious threat to the United States," Rice said. Mr. Bush decided to continue the Clinton administration strategy, and retained Tenet, Clarke and FBI director Louis Freeh from the Clinton era.
After becoming president, Rice said Mr. Bush's daily intelligence briefings touched on al Qaeda at least 40 times, occasionally in response to a question he or an aide asked.
Rice said while the Bush administration had other foreign policy priorities — like changing "an Iraq policy that was making no progress" — it also began to
"develop a new and comprehensive strategy to eliminate the al Qaeda terrorist network"
"President Bush understood the threat, and he understood its importance," she said. "He made clear to us that he did not want to respond to al Qaeda one attack at a time. He told me he was "tired of swatting flies.'"
Rice said the new strategy — calling for diplomatic efforts, covert action and the preparation of military plans — was developed in the spring and summer of 2001 and approved by the president on Sept. 4, 2001.
It "was the very first major national security policy directive of the Bush Administration — not Russia, not missile defense, not Iraq, but the elimination of al Qaeda," she said. Rice indicated the Bush team picked up on ideas that had languished under the Clinton administration.
Inissued during earlier hearings, the commission has already said that both the Clinton and Bush administration's pursued mainly legal and diplomatic ways of beating al Qaeda, and failed.
Commissioners want to know whether the administration acted reasonably in 2001 as intelligence warnings increasingly pointed to an impending terrorist attack. Rice said that when threats increased in the spring and summer of 2001, the U.S. went to a high state of alert.
But the threats were "not specific as to time, nor place, nor manner of attack. Almost all of the reports focused on al Qaeda activities outside the United States, especially in the Middle East and North Africa," Rice said.
Specifically, she said the president's Aug. 6, 2001 daily briefing did discuss the threat of a hijacking, but referred to older intelligence and did not say that airplanes might be used as missiles.
Rice did not apologize to victims' families, as Clarke did. She thanked them for assisting the commission.
Of the day of the attacks, Rice said, "I will never forget the sorrow and the anger I felt. Nor will I forget the courage and resilience shown by the American people and the leadership of the president that day.
Rice's sworn testimony could have significant implications for Mr. Bush's, which relies to a great extent on his national security credentials.
But White House spokesman Dan Bartlett said the significance of the hearing was being overstated.
"We don't believe the stakes are as high as people are suggesting them to be," he said.
Appearing on morning television news shows Thursday in advance of the hearing, commission co-chairmen Thomas Kean and Lee Hamilton promised vigorous questioning but also pledged to be fair.
"We're going to give some very searching difficult questions for Dr. Rice," Hamilton, a former Democratic congressman from Indiana, said on the CBS News Early Show.
"We're not playing a game of gotcha here with any of our witnesses," he added.
For relatives of those who died in the attacks, Rice's appearance held the promise of providing answers to searing questions, like how the terrorists got into the country and onto the planes.
"Her testimony will either undermine our confidence in this administration or bolster it," said Patty Casazza, whose husband was killed at the World Trade Center.