In one of several passages recounting her differences with the president during her time serving at his secretary of state, Clinton said she favored arming rebels early in Syria's civil war.
Clinton's long-awaited book "Hard Choices" -- seen as an unofficial start to her expected presidential run -- is set for a June 10 release.
But CBS News, whose parent company owns the book's publisher Simon and Schuster, obtained a copy and leaked several passages yesterday.
The memoir touches on the Iraq war, the Benghazi attack, Washington's ties with the Kremlin and Russia's invasion of neighbor Ukraine, as well as fallout from the Arab Spring.
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The Syria episode illustrates a key break between Clinton, who lost the Democratic nomination to Obama in 2008, and a president whose popularity has plummeted in his second term.
"Wicked problems rarely have a right answer; in fact, part of what makes them wicked is that every option appears worse than the next. Increasingly that's how Syria appeared," she wrote.
"The risks of both action and inaction were high, (but) the president's inclination was to stay the present course and not take the significant further step of arming rebels," she added.
"No one likes to lose a debate, including me. But this was the president's call and I respected his deliberations and decision," she wrote, according to CBS.
"In this case, my position didn't prevail," she wrote.
Republican national security hawks like Senator John McCain have blasted Obama for delaying military aid to the rebels, saying such waffling has emboldened US adversaries around the world.