Hillary Rodham Clinton has used her strongest language yet to admit her 2002 Senate vote backing military action in Iraq was a mistake.
The former secretary of state and potential 2016 presidential candidate writes in her memoir: "I got it wrong. Plain and simple."
Clinton's admission in the book Hard Choices is significant considering the criticism she received from the Democratic Party's anti-war activist base during the 2008 presidential primaries.
Throughout the 2008 campaign, Clinton struggled to strike the right tone rhetorically about Iraq and over time distanced herself from the vote, although she stopped short of saying she regretted casting it.
Clinton's chief rival, Barack Obama, opposed the Iraq war from the start. He gave a speech in 2002, when he was still an Illinois state senator, opposing military action in Iraq.
In a late 2006 interview on NBC's Today show, Clinton said: "Obviously, if we knew then what we know now, there wouldn't have been a vote." She added: "And I certainly wouldn't have voted that way."
A few months later, addressing a meeting of the Democratic National Committee, Clinton said: "If I had been president in October of 2002, I would not have started this war … If we in Congress don't end this war before January 2009, as president, I will."
In the new book, she writes: "Many Senators came to wish they had voted against the resolution. I was one of them.
"As the war dragged on, with every letter I sent to a family in New York who had lost a son or daughter, a father or mother, my mistake became more painful."
She continues: "I thought I had acted in good faith and made the best decision I could with the information I had. And I wasn't alone in getting it wrong. But I still got it wrong. Plain and simple."
Clinton's memoir is released on Tuesday, when she launches a national book tour and sits for a series of network television interviews. But CBS News published excerpts on Thursday. A Clinton spokesman refused to comment or verify the contents.
The political world has been eagerly awaiting the book for clues about her 2016 intentions, but CBS reported that Clinton does not write much about her political future.
On page 595, CBS reported, Clinton writes: "Will I run for president in 2016? The answer is, I haven't decided yet."
Clinton also writes about Russian President Vladimir Putin and her role in the "reset" of US-Russia relations; the raid that killed al-Qaeda leader Osama bin Laden; the September 11, 2012, terrorist attacks in Benghazi, Libya; as well as her relationships with Obama, Vice-President Joe Biden and other administration officials.
Clinton, who made several trips to Russia, describes Putin as "thin-skinned and autocratic, resenting criticism and eventually cracking down on dissent and debate".
On Syria, Clinton writes that the civil war there has been a "wicked problem" and that US officials saw no simple solution.
She writes that she wanted to arm and train Syrian rebels, but that Obama's "inclination was to stay the present course and not take the significant further step of arming rebels".
Clinton continues: "No one likes to lose a debate, including me. But this was the president's call and I respected his deliberations and decisions."
Clinton writes about her evolving relationship with Obama, describing a secret meeting with her former rival before the 2008 Democratic National Convention.
"We stared at each other like two teenagers on an awkward first date, taking a few sips of Chardonnay," she writes. It was a time, she writes, to "clear the air".