"I know we have made tactical errors, thousands of them, I'm sure," Condoleezza Rice told a gathering of 200 foreign policy experts, local officials and journalists organised by the Chatham House foreign policy institute in England on Friday.
"This could have gone that way, or that could have gone that way. But when you look back in history, what will be judged is: Did you make the right strategic decision?"
Rice also said her country harbours no desire to be the world's jailer.
Rice was in Britain where her official visit took her to the northern town of Blackburn, the constituency of Jack Straw, the foreign secretary.
Speaking at a football stadium, Rice said: "We want the terrorists that we capture to stand trial for their crimes. But we also recognise that we are fighting a new kind of war, and that our citizens will judge us harshly if we release a captured terrorist before we are absolutely certain that he does not possess information that could prevent a future attack."
Straw announced last week that Britain would take up the case of a British resident held at the Guantanamo.
He said the government would intervene on behalf of Bisher al-Rawi, 37, a native Iraqi and British resident who was arrested in Gambia three years ago.
Straw did not explain why Britain was taking up the case, but al-Rawi's lawyers have claimed that before his arrest, he was supplying information to British intelligence about Abu Qatada, a radical London-based preacher.
Previously, Britain has only taken up the cases of citizens, winning the release of all nine it has fought for.
The British prime minister, Tony Blair, has avoided criticising the indefinite detention of terrorist suspects at Guantanamo, merely calling it "an anomaly" that sooner or later must end.
Upon her arrival Rice was met by demonstrators calling her a war criminal and telling her that she should "go home".
About 200 protesters at a Blackburn school voiced their opposition to Rice's visit, some carrying signs that read: :"How many lives per gallon?" and "Blood, Lies, Oil, War."
Rice did not prove to be a star
with some locals
Blackburn, a city of about 140,000, has the country's third-highest Muslim population, and Rice had planned to visit the Masjide Al Hidayah mosque, but anti-war protesters presented a security threat, said Ibrahim Master, a mosque official.
A prominent local poet and actress also pulled out of planned appearances at a Liverpool Philharmonic concert Rice is to attend later Friday in protest against US foreign policy.
But Rice said she wasn't surprised by the protests, calling it an essential element of a healthy democracy.
"People have the right to protest. That's what democracy is all about."