But one commissioner questioned whether a pre-Sept. 11 warning gave hints of the coming attack.
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."
Commissioner Richard Ben-Veniste engaged in a testy exchange with Rice over one pre-Sept. 11 intelligence report — the president's Aug. 6, 2001 presidential daily brief.
Rice said the briefing was prepared because the president wanted to know about possible threats to the United States, amid an increase in threat reports pointing to attacks overseas.
Rice said the reports did discuss the threat of a hijacking, but took an historical view and did not say that airplanes might be used as missiles.
"It did not warn of attacks inside the United States," Rice said. "It did not in fact warn of any coming attacks inside the United States."
But Ben-Veniste pointed out that the title of the report was "Bin Laden determined to attack inside the United States." Prompting a crackle of applause, Ben-Veniste asked if Rice had told Mr. Bush that the FBI believed there were al Qaeda cells inside the United States.
"I really don't remember whether I discussed this with the president. I don't remember that the al Qaeda cells were something we were told we had to do something about," she said. "The president knew that the FBI was pursuing this issue. The president knew the director of the CIA was pursuing this issue."
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.
Clarke has also asserted that Mr. Bush pressed his aides to link the Sept. 11 attacks to Iraq.
"It was a reasonable question to ask whether Iraq could have been behind this," Rice said. "There was a discussion of Iraq. It was raised, I think, by (Secretary of Defense) Don Rumsfeld. It was pressed a bit by )Deputy Defense Secretary) Paul Wolfowitz."
Rice said not one of Mr. Bush's advisers supported any action against Iraq. But the president asked for contingency plans against Iraq in case Saddam Hussein "tried to take advantage" of Sept. 11 or was determined to have been behind it.
"I'm quite certain that the president never pushed anybody to twist the facts," Rice said.
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.
Rice's sworn testimony could have significant implications for Mr. Bush's, which relies to a great extent on his national security credentials.
Rice did not apologize to victims' families, as Clarke did. She thanked them for assisting the commission, and discussed her own feelings of anger over the attacks.
"I've asked myself a thousand times what more we could have done," she said. "If we knew that an attack were coming on Washington or New York, we would have moved heaven and earth to try to stop it."