As president, Barack Obama reversed a plan to punish Syrian dictator Bashar Assad’s chemical weapons attacks and then gave a “distorted” account of the decision, according to a top adviser’s new memoir.

“The way Obama characterized what had happened also seemed to underestimate the negative, longer-term effects of the red-line events, which I thought damaged his credibility as president and undermined the influence of the United States,” former U.S. Ambassador to the United Nations Samantha Power writes in her book The Education of an Idealist.

Power, 48, served as the top U.S. diplomat at the U.N. throughout Obama’s second term, including the weeks of debate over how to respond to Assad’s use of chemical weapons in the Syrian civil war. She recalls being surprised by Obama’s initial decision to order a military response and an even greater shock at his last-minute change of course. But it was his retrospective defense of the policy in an interview that provoked a private rebuke.

“After reading his comments, I sent him an email, urging him to reconsider the ‘distorted’ account in The Atlantic of what had happened in August and September of 2013,” Power writes.

An attack Aug. 21, 2013, which killed an estimated 1,400 people, took place “a year to the day, Washington time,” from the moment Obama had called the use of chemical weapons a “red line” that would have “enormous consequences.” He was poised to impose those consequences, Power recalls, because he “had concluded that the costs of not responding forcefully were greater than the risks of taking military action.” But when the opportunity to strike presented itself, he stunned his national security team by deciding to ask for congressional approval.

“And so for me to press the pause button at that moment, I knew, would cost me politically,” Obama told the Atlantic’s Jeffrey Goldberg. “And the fact that I was able to pull back from the immediate pressures and think through in my own mind what was in America’s interest, not only with respect to Syria but also with respect to our democracy, was as tough a decision as I’ve made — and I believe ultimately it was the right decision to make.”

Ben Rhodes, Obama’s deputy national security adviser, affirmed those comments by emphasizing that the president “was completely at ease” with the historic decision. But Power regarded that independent image as a façade.

"Obama’s retrospective embrace of pulling back from his planned air strikes seemed to me a defensive overstatement — one that I believed derived from years of being personally blamed for the carnage in Syria,” she writes. “In my experience, President Obama sounded most defiant in public when he felt the most internally conflicted.”

Power, who worked for Obama when he was in the Senate before following him into the executive branch, sent the president a private email urging him to walk back those remarks.

“When you decided to go to Congress in August of 2013, you did not expect or want to fail,” she wrote him. “By getting Congress behind you, you expected to have a thicker base to sustain an intervention that was unpredictable.”

Obama touted his willingness to resist “the machinery of our national-security apparatus,” but Power portrays him as a firm proponent of the strikes. He had planned to order the airstrike on Aug. 25, but U.N. Secretary-General Ban Ki-moon refused to withdraw a team of U.N. inspectors from Syria until they had completed an investigation of earlier chemical weapons attacks. Obama thought it was unnecessary, but he worried that Assad would seize the inspectors if the attack took place while they were in the country, so he felt his hands were tied for days.

“Obama was seething with frustration,” Power writes.

But Obama changed his plans almost the moment the U.N. inspectors left Syria. When White House national security adviser Susan Rice broke the news to Power, the stakes were clear to both of them.

“He will fight like hell to get this authorization,” Rice said, in Power’s telling. “He is betting his presidency and our reputation in the world on this.”

Power portrays Obama as motivated at the decisive moment by a combination of confidence that he would win the congressional vote and fear that Republicans “would attack his presidency as lawless and illegitimate" (and perhaps even file for impeachment) if he ordered the airstrikes unilaterally.

“Reversing ourselves in public left us looking confused, and it exposed how constrained the president was domestically on foreign policy,” she writes.

Power acknowledges that early in her tenure as a Cabinet-level official, she was too timid in airing her objections.

“Within just a few weeks, I would feel comfortable asserting a view in cabinet discussions on any subject,” she writes. “But that fateful day, less than a month after assuming my new position, I felt that I had just parachuted into a conversation mid-sentence.”

And so a Cabinet populated by three former senators (Vice President Joe Biden, Defense Secretary Chuck Hagel, and Secretary of State John Kerry) stoked a misguided consensus that Obama would be able to muster the support of a congressional majority. When that effort died, so too did Obama's interest in conducting a punitive strike.

“I did not think we could call the chapter a proud one in the annals of U.S. foreign policy,” Power concludes.