While the Pentagon and the White House’s national-security apparatuses were still moving toward war (John Kerry told me he was expecting a strike the day after his speech), the president had come to believe that he was walking into a trap—one laid both by allies and by adversaries, and by conventional expectations of what an American president is supposed to do.

Many of his advisers did not grasp the depth of the president’s misgivings; his Cabinet and his allies were certainly unaware of them. But his doubts were growing. Late on Friday afternoon, Obama determined that he was simply not prepared to authorize a strike. He asked McDonough, his chief of staff, to take a walk with him on the South Lawn of the White House. Obama did not choose McDonough randomly: He is the Obama aide most averse to U.S. military intervention, and someone who, in the words of one of his colleagues, “thinks in terms of traps.” Obama, ordinarily a preternaturally confident man, was looking for validation, and trying to devise ways to explain his change of heart, both to his own aides and to the public. He and McDonough stayed outside for an hour. Obama told him he was worried that Assad would place civilians as “human shields” around obvious targets. He also pointed out an underlying flaw in the proposed strike: U.S. missiles would not be fired at chemical-weapons depots, for fear of sending plumes of poison into the air. A strike would target military units that had delivered these weapons, but not the weapons themselves.

Obama also shared with McDonough a long-standing resentment: He was tired of watching Washington unthinkingly drift toward war in Muslim countries. Four years earlier, the president believed, the Pentagon had “jammed” him on a troop surge for Afghanistan. Now, on Syria, he was beginning to feel jammed again.

When the two men came back to the Oval Office, the president told his national-security aides that he planned to stand down. There would be no attack the next day; he wanted to refer the matter to Congress for a vote. Aides in the room were shocked. Susan Rice, now Obama’s national-security adviser, argued that the damage to America’s credibility would be serious and lasting. Others had difficulty fathoming how the president could reverse himself the day before a planned strike. Obama, however, was completely calm. “If you’ve been around him, you know when he’s ambivalent about something, when it’s a 51–49 decision,” Ben Rhodes told me. “But he was completely at ease.”

Not long ago, I asked Obama to describe his thinking on that day. He listed the practical worries that had preoccupied him. “We had UN inspectors on the ground who were completing their work, and we could not risk taking a shot while they were there. A second major factor was the failure of Cameron to obtain the consent of his parliament.”

The third, and most important, factor, he told me, was “our assessment that while we could inflict some damage on Assad, we could not, through a missile strike, eliminate the chemical weapons themselves, and what I would then face was the prospect of Assad having survived the strike and claiming he had successfully defied the United States, that the United States had acted unlawfully in the absence of a UN mandate, and that that would have potentially strengthened his hand rather than weakened it.”

The fourth factor, he said, was of deeper philosophical importance. “This falls in the category of something that I had been brooding on for some time,” he said. “I had come into office with the strong belief that the scope of executive power in national-security issues is very broad, but not limitless.”

Obama knew his decision not to bomb Syria would likely upset America’s allies. It did. The prime minister of France, Manuel Valls, told me that his government was already worried about the consequences of earlier inaction in Syria when word came of the stand-down. “By not intervening early, we have created a monster,” Valls told me. “We were absolutely certain that the U.S. administration would say yes. Working with the Americans, we had already seen the targets. It was a great surprise. If we had bombed as was planned, I think things would be different today.” The crown prince of Abu Dhabi, Mohammed bin Zayed al-Nahyan, who was already upset with Obama for “abandoning” Hosni Mubarak, the former president of Egypt, fumed to American visitors that the U.S. was led by an “untrustworthy” president. The king of Jordan, Abdullah II—already dismayed by what he saw as Obama’s illogical desire to distance the U.S. from its traditional Sunni Arab allies and create a new alliance with Iran, Assad’s Shia sponsor—complained privately, “I think I believe in American power more than Obama does.” The Saudis, too, were infuriated. They had never trusted Obama—he had, long before he became president, referred to them as a “so-called ally” of the U.S. “Iran is the new great power of the Middle East, and the U.S. is the old,” Jubeir, the Saudi ambassador in Washington, told his superiors in Riyadh.

Obama’s decision caused tremors across Washington as well. John McCain and Lindsey Graham, the two leading Republican hawks in the Senate, had met with Obama in the White House earlier in the week and had been promised an attack. They were angered by the about-face. Damage was done even inside the administration. Neither Chuck Hagel, then the secretary of defense, nor John Kerry was in the Oval Office when the president informed his team of his thinking. Kerry would not learn about the change until later that evening. “I just got fucked over,” he told a friend shortly after talking to the president that night. (When I asked Kerry recently about that tumultuous night, he said, “I didn’t stop to analyze it. I figured the president had a reason to make a decision and, honestly, I understood his notion.”)

The next few days were chaotic. The president asked Congress to authorize the use of force—the irrepressible Kerry served as chief lobbyist—and it quickly became apparent in the White House that Congress had little interest in a strike. When I spoke with Biden recently about the red-line decision, he made special note of this fact. “It matters to have Congress with you, in terms of your ability to sustain what you set out to do,” he said. Obama “didn’t go to Congress to get himself off the hook. He had his doubts at that point, but he knew that if he was going to do anything, he better damn well have the public with him, or it would be a very short ride.” Congress’s clear ambivalence convinced Biden that Obama was correct to fear the slippery slope. “What happens when we get a plane shot down? Do we not go in and rescue?,” Biden asked. “You need the support of the American people.”

Amid the confusion, a deus ex machina appeared in the form of the Russian president, Vladimir Putin. At the G20 summit in St. Petersburg, which was held the week after the Syria reversal, Obama pulled Putin aside, he recalled to me, and told the Russian president “that if he forced Assad to get rid of the chemical weapons, that that would eliminate the need for us taking a military strike.” Within weeks, Kerry, working with his Russian counterpart, Sergey Lavrov, would engineer the removal of most of Syria’s chemical-weapons arsenal—a program whose existence Assad until then had refused to even acknowledge.