The US national security adviser has acknowledged there could have been better intelligence sharing before the 11 September 2001 attacks.
Rice: No 'silver bullet' that could have prevented the attacks
Condoleezza Rice, speaking under oath, said legal barriers had prevented law enforcement and intelligence agencies from pooling information.
But she insisted there was no "silver bullet" that could have prevented the attacks on New York and Washington.
Ms Rice was appearing before the commission looking into the attacks.
The White House had originally refused to allow Ms Rice to testify arguing that she was in a privileged position as a presidential adviser. But it reversed its decision last week after a political row.
"Tragically, for all the language of war spoken before September 11, this country simply was not on a war footing," Ms Rice told the panel, which is made up of both Republicans and Democrats.
"For more than 20 years, the terrorist threat was growing, and America's response across several administrations of both parties was insufficient."
Ms Rice said the intelligence "chatter" in the months before 11 September 2001 was not enough to allow for real pre-emptive action.
Ms Rice said that one intercepted message spoke of "unbelievable news coming in weeks", while another said: "Big event, there will be a very, very, very big uproar".
"Troubling, yes. But they don't tell us when, they don't tell us where, they don't tell us who, and they don't tell us how," Ms Rice said.
She told the panel: "In hindsight, if anything might have helped stop 9/11, it would have been better information about threats inside the United States."
She said that had been made difficult by structural and legal barriers that prevented the collection and sharing of information by law enforcement and intelligence agencies.
"We have made changes since then," she said, citing in particular the creation of the Department for Homeland Security.
Ms Rice was pressed repeatedly and aggressively by several commission members on whether or not there had been real warnings which had reached the president and upon which he had not acted.
Specifically, she was pressed by Richard Ben-Veniste about a memo sent to the president on 6 August 2001 whose title was secret until now.
"I believe the title was 'Bin Laden determined to attack inside the United States'," Ms Rice said.
She said the memo was an historical document, not a warning of imminent attack - but the BBC's Justin Webb in Washington says the title is a telling one.
Ms Rice's performance was assured and smooth, but it still leaves many questions, our correspondent says.
Within minutes of Ms Rice's testimony ending, the commission announced that the 6 August memo should, in its view, be declassified in full.
The panel also questioned Mr Rice about allegations made by the former White House counter-terrorism chief Richard Clarke two weeks ago, in which he accused the administration of ignoring his warnings about al-Qaeda, and of being fixated with Iraq.
Ms Rice told the panel that in the days after 11 September, the administration had discussed "doing something against Iraq", but the president's top aides had advised against it.
"I can tell you that when he went around the table and asked his advisers what he should do, not a single one of his principal advisers advised doing anything against Iraq. It was all to Afghanistan," she told the commission.
Ms Rice said President George W Bush came into office in January 2001 determined to develop a "more robust" policy to combat al-Qaeda.
"He made clear to us that he did not want to respond to al-Qaeda one attack at a time," Ms Rice said in her opening statement.
"He told me he was 'tired of swatting flies.'"
Observers say Ms Rice's evidence could be vital for Mr Bush's re-election chances.
It is also being seen as a key moment in her own political career, with some tipping her as a future secretary of state or even president.
The White House hinted it may change course and release a speech Ms Rice was due to give on 11 September 2001, but which was never made because of the atrocities.
The speech apparently stressed the need for missile defence, rather than a war on terrorism.
After giving her testimony, Ms Rice was due to spend the Easter weekend at President Bush's Texas ranch.